Easy rail math
I've found not all rail/stile bit sets cut the same depth panel groove.
If you size the stile width to the depth of the cope groove it will make the math very easy.
For example, if the profile bit cuts a groove 7/16" deep by making the stiles 2 7/16" wide, then to calculate rail width you simply subtract 4" from the total door width.
Pretty simple but thought I'd throw it out there...........