Typically you dont need to take much off to get pretty flat, I use a granite block with 220 grit glued down. Mark the surface with a marker and rub with light pressure until all the marker is gone, rotate the part to make sure you sand evenly. No way this is as precise as a surface grinder (on my letter to santa) but works pretty well. Using a grinder you need to be carefull because its easy to take more off one edge or end making it non-parrallel as discussed.