It used to be that you adjusted the tension by way of the left nut just once, then the lever throw was enough to tighten or remove wheels. But those days are long gone and the lips on forks require that you loosen and readjust the nut every time with each wheel change.

QRs vary, but generally the nut should be adjusted so the parts make contact and begin to resist at about halfway through the throw. Start there, and fine tune trial and error, so it needs reasonable force to close the lever fully. Note that top dead center on the cam is 10-15 degrees before the full closed position, so it'll actually get easier in the end. That's what keeps it closed - the lever has to get a bit tighter before it can open.