Others beat me to it. Do what works for you.
There are plates and then there are plates. Distance from back can be varied to accommodate different plates. If the shelf is custom for a specific purpose the height at which it will be mounted can affect your decision as well. If the groove is too far from the back the plate will be angled up and won't show well if the shelf is hung very high.
The groove isn't critical. Reach in your box and grab a bit you think will work OK.
"Folks is funny critters."
Think for yourselves and let others enjoy the privilege to do so, too. ~Voltaire