What caliber/what rifle we talking about here?
I'm not sure I would cut a slit in my brass... that would make extraction tricky, I think.
Are we talking .308? I limit myself to 2.800" COAL for reliable magazine feed in my M1A. If you want to seat further forward due to having a longer magazine well and the ability to do so (bolt action probably), you can re-size the case neck and seat a bullet to about 2.950" or so COAL, deliberately neglecting to crimp the bullet in. Work the bolt slowly and easily to closed position with this dummy cartridge, then extract it and measure. You should see little marks on the face of the bullet where it began to engage the lands of the rifling.