Inboard area of a master cylinder.

[Otto-813]

Im reading a brake systems book and have gotten into the hydraulic gain section. Although I can figure out the gain ratios with the data thats in the book there is one part they leave n/a on the master cylinder. Heres an example of what it gives



Master Cyl dia/0.75in inboard area/n/a in sq. effective area/0.44in sq.
Caliper dia 2.00in inboard area/3.14in sq. effective area/6.28in sq.

Now from that data the n/a number is right around .585 but where did they get .585 from?