Since I've said all of that above, before starting the two collar test, put the level across the top of the bed of the lathe. On most all lathes, the level will set on top of the vee's of the bed. The top of the vee's at the headstock and at the end of the bed behind the tailstock will more than likely have no wear on the top of the vee's. We assume this and knowing how the factory machines the bed, this is a good plane to reference from for this exercise. I don't care if you use a Starrett 98 or 199, or any other level out there as long as it is one or two grades above a carpenter's level, it will work! The bubble is accurate enough on a 98 to detect a .001" movement in 6" easily. Just have to have patients here. And heat from your hands will have affect on the measurement you get, especially if you are using a 199 level. (Someone walking across the floor will cause the bubble to move on the 199 level!) So pay attention to what you are doing here. And make sure to mark your level with a Sharpie where it sets on the vee's. This is important so when you move the level from the headstock end to the tailstock end that it is put in the same place on the vee's. (Lathe beds with center legs are out of the scope of things here for now) Once you have assured yourself that the bed is leveled for "twist" (we don't care if the bed is running up hill or down, this is up to the owner.), you can proceed to the two collar test.