Thanks... but I cheated a bit and used one of these.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
It's handy because of the mechanical twisting back & forth if you hold the gizmo stationary with the drill running. Then you just loosen your grip for a moment to let the valve rotate a bit then tighten your grip to shift to the back & forth motion for lapping.
7 of the valves came out perfect on the first pass. One didn't have a uniform surface, it looked like two fine parallel rings on the valve surface. That one I did did a second time for about 30-40 secs and it became just like the others.
I switched hands on that valve, so I must have provided consistent pressure.
Also, you need to dip the valve stem in oil to lube the guide during the process.