Guysa, I know its called levelling. But really i think it's overrated and does not achieve much. The final test is how it cuts.
Having spent most of my working live on merchant ships, almost all of which had a decent workshop that included a lathe and sometime a milling machine, plus m,any other nice toys. for those who have never been on a ship, you can at least i imagine that a ship rolls and pitches in the seaway. Most cargo ships are not stabilised like cruise ships are, so the rolling can be quite heavy, and the pitching can be quite violent in heavy weather. So how to level the lathe?.
OK, so when the ship is built a very heavy steel frame is made where the lathe will be installed This frame is made so stiff that it wont flex and is then mounted to the ships frame with flexible mountings.
The lathe is then placed on the frame and bolted to it using shims and or jacking screws so that the lathe is true to itself, the centerline from head stock to tail stock must be parallel to the ways, then test cuts are made and the final adjustments are made in the normal manner.
Once the lathe is settled in this way it should never need adjusting. I have been able to do good turning work on such a lathe in heavy weather with the ship rolling up to 37 Deg both sides and back every six seconds plus heavy pitching.
Under such conditions the only way to remain stable and have hands free to operate the machine is is to wear a safety harness and be secured in the working position. Make sure the job is secure, you don't want it flying out at you and you can't move.
I have secured my lathe in a similar manner. the steel frame sits on the concrete floor , the frame is stabilised and the lathe is adjusted true to itself within the frame. theoretically I could drag it to another position and although it might rock a little on the floor. it should not upset the lathe at all. If it rocks a little all i have to do is slip in a couple of wedges, who cares if it's not truly level, I don't and nor does my lathe.