It's a bit late now but the way to avoid that outcome is to keep dropping the valve back into the guide, holding it down with a fingertip, and measuring the tappet clearance. You just keep doing that after a second or less of light-pressure stem grinding each time.
I don't recommend cutting (or, heaven forbid, lapping) the seat deeper by anything like that amount anyway. The best answer in my opinion is to buy a new, or better, find a second hand, valve.
