Hand lapping valves

I am assuming (and maybe assuming too much) that the exhaust valve and seat are cut at a 1 degree difference in angle so that the contact when new is just at the outer edge of the contact between them. If that is the case, then a thin line at the outer edge is all you should get.

I have always assumed this is done on the exhaust side to keep the hot gases in the combustion stroke away from the main contact face of the exhaust valve, as it runs a lot hotter anyway, and this would help avoid burning through the exhaust face/seat. (But corrections to that thinking may be in order!)