I say if you need an explicit computation with nonintegral coefficients, you shouldn’t be working in that area anyway.
For a short 45 degree offset: Take the length, in inches, of the offset: add half of that number to it, then subtract 1/16″ for each full inch.
To convert inches to decimal feet: 0“ is 0.00 feet 3” is .25 feet, 6“ is .50 feet, 9” is .75 feet, 12 inches is 1.00 feet. Select the closest one of these, and then add or subtract .01 feet for each 1/8th inch. To convert decimal feet to fractional inches, select the closest quarter and then add or subtract 1/8th inch for each .01 foot above or below.