I was thinking of something that I use for a general rule of thumb:
Conventional machines: That would include your lathe, other lathes, regular mills, standard surface grinders, standard cylindrical grinders etc.:
.0002" per 12" with 12 to 20 PPI (Points per Inch) with 50% contact.
Precision Machines: That includes all CNC Machines like VMC, HMC anything with a ball screw in general and Jib Bores,jig grinders, Studer and Drake type Cylindrical grinders, surface plates, Straight-edges, etc. .00005 per 12" 30 to 40 PPI with 50% contact.
The spec's are for scraping and alignment. I was going to include squareness too, but we leave some area's out of square on purpose to compensate for tool pressure, sag, wear as RC mentions. This technique to concave the shorter bottom side can be tested using a precision level setting in the table top and as you push the table out you watch the bubble to check for sag or if the table bends. I use this mostly on Excello boring machines or Heald ID Grinders that have thin and long tables the bend like a noodle.
Also remember to leave the center 40% of one side, it depends and not always the short or moving slide to compensate for wear because the dirt works it way in under the wipers.
You have to investigate and think about which side. Like the bottom of a short table of a long bed grinder would be relieved. Or bottom of a short bed grinder with a long table moving over it. One needs to check to be sure if the machine has long travel that the moving part does not fall into the relief. This amount varies depending on the machine. But for the most part scrape the high spots in the middle 2 to 5 scrapes or .0004" to .001" low. Some machine builders relieve with a end mill and it might be low by .030" I usually don't do that because dirt and coolant can work its way in. If you ever need help figuring this out please write me as sometimes I miss questions asked on here. Rich
Edited: The info on relief of 40%. and concave scraping.