Plus take into account the frictional resistance of the pad and disc combinations you are using. Relative areas alone won't give you a useful figure (for example you could have massive pistons, lots of clamping force and huge swept areas but pour oil on them and they'd be useless). It's not an easy sum to get right. I'd go and talk to someone like Girlock or maybe even Brembo or other brake specialists for some insight into this. Girlock did a great job for me when I put more modern brakes and a new twin booster system on my Maserati.
There is a chance you might be over-analysing it too.
CP_e Standback & PNP; CP_e 3" SS Downpipe; Corksport FMIC with Top-mount K&N filter & OEM Ram CAI; Turbosmart BOV; Dashhawk; Prosport Boost Guage; JBR solid shift bushes; DBA 4000 Wiper-Slot front rotors; Hawk Ferro-Carbon HPS Street front brake pads (@ 69,000km); Sumitomo HTRZIII's in 225/45 x 18