Tire Size X 3.14 (Pi) = X
And then
Spur / Pinion = Y
X/Y= Rollout. Thats what i use and it seems to work pretty well.
In stock i usually run a lower rollout then usual because of the small tire size, i was running around 38.5. But i usually like to go in between 38-43 rollout.
In 19t i tend to just keep the same pinion gear as stock and runs very nicely also. Its all trial and error and the key is tire size in my mind.
Jon