When I level my SB lathes I go off the top of the V ways. I have a 12" level that spans them. I mark where I'm going to repeatedly place the level with a marker and then very lightly hone the flats on the V way tops in this location to verify there are no high spots. The tops of the V ways should not have any wear at all since they shouldn't come in contact with the mating surfaces (there's a clearance there). Just make sure there are no bangs, burrs or git there.
I would never use the carriage and the flat ways can have a lot of wear on them.
My two cents,
Ted