When working with a bed level, sometimes you find that your bed is level but it is not flat. Mine, for example, is raised in the center. Because of this, it is impossible to use a standard bed level procedure. You need to use a mesh bed level. This feature doesn’t come enabled on all printers, I had to update my firmware to be able to do a mesh bed level. How a mesh bed level works is it breaks the bed down into smaller “tiles.” On mine, the tiles are approximately 60mm x 60mm. This allows the compensation to follow the contour of the bed instead of assuming that the bed is perfectly flat. As I always say, don’t try to solve a mechanical problem with software. Level your bed first, as best as you can. Then incorporate the mesh bed level. Make sure that you heat up your bed for 15-30 minutes before you do the mesh bed level so that your bed will take the shape it will be in while printing. Store your bed level profile and make sure that you activate it before you start printing. I typically do a new mesh bed level in the summer, in the winter, and anytime that something significant changes on my printer.
So, what’s a good amount of deviation for a mesh bed level? A good general rule of thumb is 0.1mm. If you can get your deviation at 0.1mm or better, then (for hobby purposes) you are doing pretty good.