I fixed the focus (and a few other things) on my NEC MultiSync II recently. I think the focus drift is an ageing thing, and you'll want to let it warm up really thoroughly (perhaps an hour) before making the final adjustments. Mine is now a bit blurry and yellow at power-on, but looks great when it's warmed up.
I think it's fairly typical for the focus to be the upper adjustment on the flyback transformer, and the screen (grid 2 voltage) to be the lower one. I used a visor magnifier to get it dialled in. You might find that the convergence improves when the focus is set right.
