1) Begin by measuring from the center of the wheel to the inside of the rim. See below.
In the above, the radius is 12 3/8 (12.375) inches
2) Use the formula: Spacing = 0.313 x Radius. For the above, 12.375 x 0.313 = 3.87 inches, or 3 7/8 inches.
Marks will be 0, 5, 10, 15, 20, 25, 30, 35, 40……90, 95, 0. If you read 128 on your cyclocomputer and 35 on the rim your complete reading is 128.35.
3) Mark a zero point.
4) Mark 25, 50, 75 also. These are easy, as they are quarter points and need no fancy calculation.
5) Lay out the marks between the quarter points as below. You will be dividing that interval into 5 equal lengths, using 4 marks. When you are done, the remainder will be the same length as the other intervals.
6) Repeat for the other three-quarters of the wheel.
When you are done your wheel will have decimal markings.
Commentary invited.