I have a old Cincinnati 15X60 lathe.. I use a Machinist level on the ways to make sure they are exactly parallel ... if there is any twist in the ways it will cause the cutting tool to cut a taper rather than a straight even cut. The level I have is accurate to .0005"/10 inches By taking readings from near the chuck down to the tail stock and adjusting the legs, you can adjust any twist out of the heavy cast Iron. Just adjust a bit, then wait awhile, then measure again... until you have it perfect.
