I would say use all of it. Build a good sized berm at the end and move your firing line and/or target stands to suit your needs. If you build it short, that will guarantee you'll eventually wish you'd built it longer.

I kinda thought that's what you guys would say.  I'll do that then.   

I've got about 300 tons of fill in a big-ass pile on the end of my lot, that I've formed into a huge U shape for the berm,  Just gotta lay down a layer of screenings for a table/ shooting position .