Ok so I'm using a fret calculator and it's pouring out numbers like 26.64mm.

Do I just take a normal ruler and try to measure as close as possible then take a saw and route a slot?
you need something that measures milimeters. a normal ruler does not do that. an architect's ruler does, i believe. you'll have to find out how to use one, though, they're not easy.

for frets, you need to be super spot-on, or it'll sound like crap.
Unless it's done by a computer, there is no way you can place a fret to that accuracy. Round it off to one decimal place, and then mark your fret as close to that as you can get. If you're good, you will be only a few cents off either way, and you have to have a REALLY good ear to hear that little difference.
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