Good point. I should have added for the OP that the panel should be cut
1/8" narrower than the rails (not exposed rail dimension, OP, the total
rail length including tongues). That means width of door, minus 4-1/8".
For the height of the door panel, subtract 4-1/8" from overall height of
door (stile length). This applies only to straight rails; not
cathedral, etc.

Dave

Jody wrote:
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