Also, it matters how wide you want the flange to be. A lot of combinations are possible. I am making one now, with 1.75 flange and 1.25 background.
If you can do work out the math for one, then you can figure it out for other sizes and won't be restricted to one combination.
It goes like this (for my current one):
1.75 F + 1.25 B = 3" - .5 for the seam = 2.5 (1.5 F + 1" B left)
Pressing it in half, you get 1.25 per side.
The back side is all 1.25 F. The front is the .25 F that's left over and the 1" B, to equal 1.25.
From there, you subtract your seam allowance, to get the final numbers per side.
charlotte, hoping this makes sense