I use fresh raw milk as well, and I've found that I need about 2ml or 3/8 tsp of rennet per FOUR gallons of milk to get a 10-12 minute flocculation. I formerly just went by the bottle, and didn't time flocculation, and experienced hard, tough, rubbery, and/or overly moist cheeses. So probably cutting down to a 1/4 or scant 1/4 tsp will put you closer in the right ballpark rennet- and flocculation-wise.
As I understand it, the reason that longer coagulation makes for a moister cheese is that the casein network has more time to form more and stronger bonds - which later on better hold or capture more whey. Hence, a higher yielding, moister cheese.