Measure twice; cut once. You have to abide by this tradesman’s law when sanding down the saddle on your guitar. You can always take more off, but you can’t put it back on and no one wants a shimmed bridge saddle. This string height ruler will give you an accurate measurement of ...
The frets are supposed to be level with each other (they should all be the same height). There is an exception to this rule (upper fret "fall-away"), but we won't be getting in to that here. When the frets are not level with each other, that means some of the frets are shorter...