If you have to mount it further to the rear, you'll just need to move your head farther back on the stock. I have mine mounted so far forward because I go nose-to-charging handle.
At the optimum eye relief, the black "ring" that separates what you see through the scope from what you see around the scope is thinnest. If you move your head forward, it gets thicker and you lose some peripheral vision. If you move rearward, the field of view through the scope gets smaller. I could actually stand to move it a little further still, to fully minimze the "black ring".. but this is a good compromise which allows my head to be back somewhat from the charging handle if the shooting position requires it.
The RM3G match was a BLAST. If you like shooting your AR, pistol, and shotgun in "field" courses, I highly recommend you go next year.
-z