No need for a lathe for trueing a crank, in fact this "between centers" logic may look impressive and scientific - but isn´t . That way you can only guess what the crank ends may do once installed in the case but you do not know . Just get two alu plates or even two plates of plywood and cut a Vee in both to lay the crank in there. So this is like spinning on the inner bearings and you check with clock at the ends. The readings will be EXACTLY like in the engine, no guesswork from numbers you get from between questionable centers and runout on bearing seats.
Vic
Vic