You are deciding on an 'island' type layour here which can look very effective. The choice of how far back to place it would depend on what you want the back of the tank to look like... if you want lots of green in the background in the form of stem plants of vallis then you would need to leave space to do so. I personally would recommend leaving a few cms for plants at the back but would say you don't need much. Aquascapers go for miles to get the look of depth in a tank (front to back depth) so the further back the root goes the better really, just leave a little gap enough for some planting. If you don't plant it, think of it as a gap for the algae cleaner to fit in...
