That is the best way I do it. My cutters cut 7/16 so my rails and stiles
are 2 7/16. You will loose the 7/16 with the cope cut so your left with
2" on each side. Take the total door width and subtract 4" and that's
the mesurment from inside the tongue to inside the tongue. It does not
matter if your door is some odd ball size your rails will be 4" narrower
than the width of the door. Then take off another 1/16 to 1/8 for raised
panels or just a 1/16 for plywood flat panels.

David wrote:
Please understand that if your router bits cut different tongue & groove
depths, then you would need to adjust accordingly. I find his method
simple enough for a screw-up like myself to easily understand.

If you are making cathedral arches or other pattern cuts on the top
rail, of course that rail will be taller; usually around the 4-1/4"
range IIRC. Everything else gets cut to 2-7/16" widths.

Dave