where is the first interval (0.01 mm), n is the number of intervals, is the size of the last interval (1.2 mm), and is how much each interval increases per step.
We know that we need the sum of all these intervals to be 9 mm, so
So we know that
So we need 14 intervals. (If you can go over the 9 mm, you can use 15 intervals.)
Now, what is ? Well,
So your first interval is
Your second is
etc. for a total span of 8.46996 mm. (If you do this using the 15 intervals and the total span is 9.075 mm.)