Getting your measurements somewhere between 1/16th and 1/32nd of an inch is really good enough. This degree of precision - especially with fat wire nails for frets - will amaze you.
The 3 to 4 decimal places your calculated fractions represent are actually thicker than the pencil lines you will mark. Even a good mitre box will have more variation in it than these scientific measurements.
It's a box and a board. You'd be surprised at how wonderful the intonation is, on a neck that is "close enough".
Print out your fret calculations in mm.
I measure to the nearest 0.5mm, which is amply accurate enough - and a lot closer than +/- 1/4"!! Sure mm is for the kids, and everything else I do in old money, but it does make fretting a whole lot easier :o)