There is certainly a time when fret work is necessary, or even a complete refret job. Don't take this the wrong way, but your question about sandpaper/file grades suggests you may not have done enough research of the the problem.
Make absolutely certain the frets not being level are from fret wear, and not from other problems associated with the neck. Yes- you can temporarily compensate a bowed, bent, or twisted neck with a fret leveling- it ain't the right fix. You just might have to straighten up the neck, level the fingerboard, and then refret.
Here's a couple good resources for info-
www.frettech.com www.stewmac.com