One of the problems is the slab is not stationary, and if you've mucked about with this, you know it can move enough to create tapers.
On big lathes, 6, 8 10 or more feet you have no choice but to deal with it and tweak them occasionally. But on small lathe, I'd be thinking how to get it on three feet, like a Schaublin or Monarch 10ee. Monarch has three contact points under the massive cast iron stand and the stand is properly fit to the bed. Great if there is a massive CI stand scraped to the bed, but what about the style like in this thread? Schaublin mounts the bed to the tray or stand via the hemispherical washers, that fit in a matching counter sink. Two at the HD and one at the tailstock. just food for thought ..... get the lathe attached to the stand by three points, maybe via an adapter plate, and never worry about twist/levelling again