I don't have a specific formula. But this has worked for me.
Figure out how much reflex you want, and where you want it. For instance, you want 4" of reflex, starting 14" from the end.
Now divide both of those measurements by lets say 7. So starting 14" from the end, mark your form horizontal every 2". Then mark your vertical every .57". Connect the marks as per the picture.
If you want your curve to be more precise, divide your measurements by a higher number.
I draw them to scale on paper 1st to get the curve I'm after.