brent
I've never used your rennet and basically you have to just try and then adjust amounts next time. You have 2.4 litres (0.63 US gallon) total of milk & cream x 7 ml/10 litres manufactures recommended dosage = 1.68 ml. But I believe cream takes more rennet than milk, so I'd guesstimate 2-2.5 ml of your rennet if you are using pasteurized cow's milk with CaCl2, and yes I'd use 3 ml if you don;t have any CaCl2. In my experience it is not critical in Cream Cheese and similar Neufchatel recipe as rennet just aids in whey expulsion, which you will get from hanging the cheese anyway. More critical is how long you hang to allow cultures to grow.
On using ripened
buttermilk as your mesophilic starter culture, the goal is to introduce lots of "correct" energetic bacteria to get the cream cheese flavor. Thus the amount of ripened buttermilk is less important than it's strength. On amount, this
Cream Cheese Recipe says to use 125 ml for 2 litres cream, and the
Neufchatel Recipe says to use 60 ml for 2 liters milk. Tea's recipe you quoted is roughly 1/2 way between these two, so I guesstimate 100 ml of highly ripened buttermilk, (freeze the rest for later use).
Hope helps and let us know your results, problems and all if want like I do
.