Playing with brakes: sizing a system

What am I doing wrong here? 

I am trying to check my work against the known fact and then look at what it could be with brake upgrades:

Currently the car runs the stock brakes so I was able to find the piston diameter and convert them to area, master cylinder included. 

now I plan to do a brake upgrade/replacement in the near future and I am wondering should I change my master as well. There is an update that I can do which goes from a .875" bore master cylinder to 1" that is on the table but the main goal is replace the floating single piston stock rear caliper to Z32 rear brakes which are fixed mounted, 2 piston. each piston is a smaller diameter slightly. 

problem is I cannot get to the stock pedal ratio math using the stock parts. I summed the piston area (8 pistons in the front, 2 in the back (stock) or 4 in the back (z32) and my pedal ratio is far away from what is listed. I should just divide the total piston area by the master cylinder area to get my pedal ratio, or am I missing something?

Where are our braking gurus? 

only count the piston area that's on one side of the rotor.  that'll get you closer.

More words: on a sliding caliper (piston on one side), you use the full piston area. On a fixed caliper (pistons on both sides), you use half the piston area.

In reply to Keith Tanner :

I was wondering how that was to be accommodating. I was only thinking of using once per side in my calculation, so I need to halve that because im counting 2 full pistons rather that a caliper sharing 1/2 of a piston. 

my 8 count front really is 4 then. 

but its still then 23:1/16:1 window, very far from 2.8:1

What are you calling pedal ratio? I've only heard it as the actual ratio of foot to pivot vs M/C to pivot...

Every RX-7 that I have measured had a 4.5:1 pedal ratio.  Easy to redrill for a 7:1 ratio for manual brakes.