Lets say you have a few plates 8" X 9" X .250 thick. Some kind of tool steel. You need it to be flat within a tenth and to size within a tenth. And you need to get it done this year... You have some kind of wet grinder with coolant(or else forget it). If you don't have a wet grinder, go to the fish store and buy a 12$ fish punp, drop it in a bucket of coolant, rig a hose for the coolant to run back in the bucket. Use paper towels for a filter on top of the bucket.
Ok, first thing is you want to make time, so no matter what kind of mill your in, get the meat off as fast as you can, leave .025-.030 per ide. Use the biggest flycutter you have, whatever. After your done abusing it, use an endmill and in 2 passes mill off most of that .025-.030 per side, without holding it hard enough to warp it. Loosen the vice, loosen the clamps, etc...leave .007/side. Drill as needed. Mill as needed.
After heat treat because you used an endmill instead of a flycutter the plates will be flat. Very flat. As flat as you milled them.
Place a few sheets of newspaper on the chuck, throw the peice on, grind to cleanup. Flip it over on a few sheets of newspaper, grind to cleanup. Flip is over, no newspaper and grind to size.
Sometimes flipping it one more time helps especially if you heatreated it inhouse/non vacuum.