You can do this via a bit of math
The maximum terrible-ness index just makes it so that you can’t go above 2x drain rate. Ie if you make the maximum index 6, >=625 will now be 5 and result in a drain rate of 2.5, 1875 will now be 4 and result in a drain rate of 2, and you’ll need >=6875 to reach an index of 0 and drain rate of 0.25 again.