Hi bbracken677,
The amount of rennet to use depends upon how strong it is. The terms "single" and "double" strength are sometimes used, but they are not really very specific. Rennet strength is measured in IMCUs, which stands for "International Milk Coagulating Units". Keeping all other things equal, if you normally use 1/4 tsp of rennet which is 100 IMCUs, and switched to a rennet that had the strength of 200 IMCUs, then you would use half your normal amount (so 1/8th of a tsp).
Now, I have some store bought liquid rennet (Renco), which is around 65 IMCUs, and to set 10 L of milk I use about 6.5 ml of the stuff (that's just over 1 full tsp 1 tsp = 5ml in NZ). I also have some calf rennet, which is stronger (280 IMCU), and I use about 1.6 ml of that, and if I use my 750 IMCU strength vegetarian rennet, I only use 0.6 ml! All these amounts set my 10 L of milk around 12-13 minutes.
In other words, without knowing the actual strength of the rennet, just saying the amount can be misleading. A pint of beer is one thing, a pint of double proof rum is definitely another!
- Jeff