Without knowing how high you want to go it's difficult to recommend a spring. You are aware that you would also have to lock the gearbox to semi auto only for a DMR? Do you know what your sites restrictions are for DMRs?
Right, Ground Zero (which I assume is a regular site of yours) limits DMR semi only AEGs to 425FPS. You'll need about an M130 spring to get you near that, but you'll need a whole host of other stuff to get it accurate.