Yes. Gianaclis Caldwell has a good description of how to do this in "Mastering Artisan Cheesemaking". I was running into this problem when I was first getting started and went through it. It was very interesting (I'll give you a brief description below). Spoilers: My problem was actually chlorine/chloramine in my water. When you dilute the rennet, you really need to you cool *non-chlorinated* water. Boiling and letting it sit, or letting it sit over night is not good enough to remove chloramine. Even though my water supply claims to use chlorine and not chloramine, it also wasn't good enough. I actually use bottled water for dilution now because bottled water is usually UV sterilised and contains no chlorine. You can also use distilled water if it's easy to get in your area.
Now on to calcium chloride and how to test if you are lacking dissolved calcium. First a bit of theory. Rennet works by "cleaving" (cutting) the "kappa casein" from the "casein micelles". In less technical terms, the milk protein we use for most cheese (casein) exists in tightly packed balls called micelles. On the outside of the micelles are a particular shape of casein called "kappa casein" (on the inside of the ball are alpha and beta casein -- all casein, but just slightly different variations on the shape of the protein).
The kappa casein on the outside of the micelle is what keeps the casein micelles apart, and fully distributed in the water of the milk. Rennet (specifically "chymosin" rennet) is an enzyme that specifically cuts those bits off. When that happens the micelles can bind with dissolved calcium and stick to each other, using the calcium as a kind of glue. I should note that this is *completely different* that how milk curdles due to acid, but I won't complicate this discussion to get into that. It's just very important to understand that rennet formed curds and acid formed curds are created differently and are actually even different chemically, even though they kind of look the same from out perspective.
Almost all of the calcium in milk is in the form of a salt called "calcium phosphate". All of the calcium phosphate in the milk is actually bound up in the casein micelles. You can think of it like covering yarn with chalk and then winding it up into a tight ball. Basically all of the chalk is on the inside of the yarn ball. Even if you dunk it in water, the water can't get at the chalk unless you loosen the yarn ball a bit.
There is some calcium dissolved in the water in the milk, though. This is what allows the micelles to bind together. However, when you heat milk, the calcium *comes out of solution* and falls to the bottom of your container (it "precipitates"). If you live in a place with hard water, you have seen this countless times. When you point (or even just heat) water, you get "scale" -- chalk and gypsum -- coming out of the water and creating a film on your shower, or on your fixtures or on the bottom of the kettle. That's because calcium salts tend to be weird: they are less soluble as the water temperature goes up (completely opposite to table salt or sugar or whatever). We add calcium chloride to milk because it is *more* soluble the higher the temperature goes (and it just has a relatively high solubility compared to other calcium salts).
When we pasteurise milk, we heat it. This causes the dissolved calcium in the milk to precipitate and so there is less dissolved calcium left in the milk. When the curd is forming, you need "enough" calcium to bind the micelles together. If you don't have "enough", then some of the micelles won't bind together. The more calcium you have, the more micelles will bind together and the stronger the curd you will get. Not only that, but the more calcium you have, the faster the process will go. You don't have to wait 20 minutes for sufficient calcium to drift by and help form the curd. However, there is a limit. When you have "enough" calcium, it will basically always be available and so it will bind as quickly as possible and as fully as possible.
TL:DR: If your curd is slow to form and you are sure that you have enough undamaged rennet in your milk, the reason is because you don't have enough calcium. If you add more calcium, it will speed up *and* form a firmer curd -- up to a point. When you have "enough", you have "enough" and it doesn't really alter things very much.
Note that pasteurisation is not the only reason for a lack of calcium in milk. Nutrition, disease and medicine can all affect the milk. For people using raw milk that they buy from a farmer, it might seem that it's impossible to have a lack of calcium. This is not true. It does happen. Also (and I hate to say it), in areas where the sale of raw milk is illegal, there is an incentive to sell raw milk that can not be sold to the normal milk pools. If you have a sick cow that's undergoing treatment and you can't sell the milk to the pool, but there is someone wanting to buy that milk illegally... well... You can see how that can happen.
Finally, how to test it. First get some milk and heat it to 36 C. Heat up a wine glass to the same temperature (just using warm water). 1 drop of single strength rennet is about 10 IMCU (International Milk Clotting Units). Basically 36 IMCU of rennet will coagulate 1 liter of milk at 36 degrees and a pH of 6.6 in about 12 minutes (or something close to that -- I always forget the exact definition). Making the numbers a bit simpler, this means that 1 drop of rennet is enough for 250 ml of milk. We're going to use 1 drop in 50 ml of milk, which means that it's 5x the strength that you would normally use. This will get it to "flocculate" in less than a minute. Put the milk in the glass, add the drop of rennet and then keep swirling it until you kind of see a "snow globe" effect. Time how long that takes.
Now do the same thing with a drop of calcium chloride in the milk. Time how long it takes. Do it again with 2 drops. At some point you will notice that the time doesn't really change much. If it coagulates quickly without any added calcium, then you don't need to add it to that milk. If you have to add any, then you should probably add about 4 drops of fully saturated calcium chloride to you milk per liter (I use 5 drops of commercial preparation per liter because I think it's not quite fully saturated for some reason). Don't add too much, because it can make the cheese bitter. You will never need more than this.
After all of that, you will probably find that you *do* need some calcium chloride (so add it if you aren't). If it's still slow, it's almost certainly because you've damaged the rennet with chlorine or chloramine. It's amazing how often that ends up being the case.
