Hang a grid paper at 100 yards and see how many inches or cm each holdover mark is from center. That will tell you the adjustment for each mark.
Next, chrony your favorite load and enter that into a ballistic calculator. By comparing the calculated drop per distance with the number of in/cm from center for each mark, you will know what distance each mark corresponds.
For example, I know for my .308 rifle, 0.5 mil = 200 yards, 1 mil = 300 yards, 2 mils = 400 yards, and close to 5 mils = 600 yards.