Can someone tell me if this is a feasable way to layout commons.

lets say its a 10/12 with a 20' span.

i look on my framing square, and it says "COMMON RAFTER LENGTH PER
FOOT RUN" for a 10/12 it says 15.62.

so i multiply 15.62 by half my span which is 10.

gives me 156.2 which is about 156 1/4"

i subtract 3/4" for my ridges, gives me 155 1/2"

now is this 155 1/2" the measurement from my birdsmouth to the top of
my rafter?

also how do i determine the depth of my birdsmouth and do i need to
factor this into my math somewhere?

if anyone can understand what i just wrote, i appreciate your help...