The thing is "it is" a crap shoot. Front and rear tires, (when you change sizes or compound), front to rear car loading (passengers, other weight, trailer?) which is why an adjustable valve is nice.
The only real reason disk systems use larger "tanks" on the master is to accomodate large amounts of PAD WEAR. Day to day operation requires very little fluid, as the calipers don't move very far.
Bore size of the master has nothing to do with front / rear brake proportioning.
