I can't really help with the dosing since both of my diabetics either were or are on Lantus. But we have an entire ISG (Insulin Support Group) here just for folks on Prozinc. And it sounds like you are off to a good start with diet.
The three big keys to treating a diabetic cat are:
1) Insulin and you are already on a nice gentle insulin
2) diet...got that one whipped already too...Fancy Feast Classics are perfect for a diabetic cat
3) Testing his BGs at home...and we can help you get started on that one. Home testing is one of the best tools you can have on managing your cat's diabetes. It is kind of a learning curve but once you get it down, there will be no more spending a ton of money at the vet's to have them run curves which are really pretty much worthless because stress raises BGs and just about every cat out there is stressed when they are at the vet's. By testing him at home with a regular human meter, and with the support of the folks here that use your insulin, you will see just how the insulin is or isn't working for him. And they can help you adjust his dose yourself at home, just like a human diabetic does.
I adopted both of my diabetics from Diabetic Cats In Need, as diabetics, neither of mine have seen the vet since I adopted them for anything other than routine stuff. My Maxwell has already been in remission for over a year now and off insulin and my newest girl is pretty well regulated as well, all thanks to the folks here.
There are all kinds of tips and tricks as well as plenty of videos on how we test at home here
http://felinediabetes.com/FDMB/viewtopic.php?f=14&t=287
Mel, Maxwell, Musette & The Fur Gang