Hmmm, rennet sounds fine. I usually just mix mine in an egg cup of water (pre-boiled, and left to cool) and put the rennet in the water just before adding the mix to the milk (add rennet to water, stir, dump into milk).
Assuming your water is free from chlorine, etc, then it seems to me that the rennet isn't the problem. Your PhD meter also sounds like your culture and progression is good. So, that only leaves your milk itself. It may be that it is heated quite high during pasturisation, not enough to be UHT (which won't really form a curd) but higher than optimal perhaps? Is your curd weak after you cut it? Does it shatter when you stir it? These are signs of milk pasturised at a temperature that is too high for good cheesemaking. The more expensive milks, and organic milks, etc, are often pasturised at higher temperatures to extend the shelf life. I usually use a fairly cheap milk here and it works fine, although the best store bought milk in New Zealand for cheesemaking that I've found is the Farm Gate Silver top, which is non-homogenized and forms a really nice solid curd. It's around 4% fat as well.
Anyway, for floc calculations, you judge it like this: add the rennet and then float your bowl. I tap the bowl a few times to get that visual of what it's like when free flowing. After 5 or 6 minutes, tap gently again (it is really more of a quick touch). Do this every minute or two. At some point, you will notice that the bowl is not as freely floating, you get the impression that the fluid it is in is thicker. Eventually, it won't move, it will just lurch forward then spring back to place, like a cartoon with its feet stuck in glue. That is the floc point. The bowl isn't stuck down in the curd, it's just no longer in a fluid, so sure, if you tap hard it will move, but you're forming a curd not cement.
If you're doing that (gentle touches, not shoving the bowl), then it seems to me you might want to try other brands of milk and see if things improve.
As for the semi-lactic crotins, just use the few drops. It takes 24 hours for the curds to form, and the rennet should just help it along.