It came to me to go a different more elegant route to achieve perfect and accurate spacing for these rails.
I always cut 3/4" dado's and groves to lock the bottom into the sides, sides and bottom into the bottom rail of the front and back face frames and the side stiles of the front and back face frames. this adds considerable strength to the carcass with no mechanical fasteners.
Anyway while cutting the bottom dados in the plywood side panels to receive the bottoms I cut additional dados at the locations that the drawer mid rails would be located. I use those dados to perfectly index the rails.
Back Face frame attached to carcass
The additional indexing dados for the mid rails.
The mid rail perfectly indexed into the side dado and notched to fit the face frame stile. Ready for the single screw to hold it into position on each end.