The amount of hot air nonsense promulgated concerning lathe levelling over the years is probably a significant contributor to global warming!
The pragmatic person ignores the whole lot and concentrates on getting the lathe set up with the bed in its natural, unstrained state.
If the wood floor is on a solid base, whether direct or with intervening joists, its not going to move unless there are serious flatness issues creating significant gaps between base and floor.
If its a suspended floor first thing to do is to check if the floor actually moves when you wander around. Adequately sensiitive level in various places with an assistant to watch it whlie you walk around will sort that. If it shifts think about where the supporting joists are and what might be done to improve support. Folk have been known to cut the whole floor area out where the lathe is going and insert a concrete block. Overkill for an itty bitty SouthBend. One or two layers of the waterproof tongue and groove chipboard underfloor stuff running all the way across screwed down to the floor should make it amply stiff. Check with your level.
If the lathe is going on a bench install the bench and screw it down with suitably strong joint brackets. Shim the feet so its as close to level as your patience will stand. Good strong bench will seriously stiffen the floor too. Sit the lathe on top, slide the bolts through the feet and use pull out force on spacers / shims under the feet to judge if its sitting evenly. If I were it do the job again I'd use a nice flat metal bench top and bolt the lathe feet to tapped holes in larger spacer feet maybe 2 inches thick perhaps 4 inches long by 2 inches wide. SouthBends are too low to the bench for easy cleaning and the wider feet makes it easier to do the shim trick. Two 1/4" or 6 mm bolts for each of the foot / spacers rather than standard one large is easier.
Designate one the master foot, rear of headstock is hardest to get at so probably one to choose. Slip a shim under so its a bit high and lightly nip up the bolts. Shim under the other feet until it takes a smooth stiff pull to shift a 1 or 2 thou test shim. Refit the test shims or make up an equal pack and bolt down all round just enough to keep it in position. 10 – 15 ft lb will be fine.
That method was good enough for sub thou errors when initially setting up a 4 section 12 ft long optical test rail with 4 feet per section so it will work just as well on your lathe as it did on my two SouthBends.
If the lathe has its own legs same sort of trick works but you have to be more creative. Frankly I'd take lathes oth this size off their legs and just bench mount. Underdrive systems usually have asolid cabinet stand so process there is to loosen the lathe, bolt the stand down solid and then do the shim trick between lathe and cabinet.
Clive