IMO running the bike till it shuts off is unnecessary. Not too many of us do that, and don't have issues with gas in oil, because normal fuel levels in the carb float bowl will not overflow onto the ground or into the inlet manifold, and if the petcock is shut off, the level will not get above normal. I don't know what the seafoam could be expected to accomplish to improve things.
The culprit is a float valve that is not fully shutting off against the head of a gastank full of gas and an open petcock. This may be only an intermittant or infrequent condition, but if the bike sits for any length of time like that, there will be significant leakage.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1996 Ducati 900SS CR (California Bike), 1973 Norton Interstate (in a box in the basement)