IRL540/30N06L Dimming Headache

Actually the basic idea is ok - but doesn't give 16 - bit. Sorry.

A better approach would be to make the resistor so as to give eg 1/16 or 1/32 the full current.

That way you could interpolate the 3 or 4 lsbs of the main power path to give 11 or 12 bits - good enough?