[DouglasCole]

I used fractional dice in order to give some bite to each extra point of ST.

the truly EASY way to do this is to realize:

+1 is 0.28 dice
+2 is 0.56 dice
+1d-1 (so from 1d+2 to 2d-1) is 0.71 dice
+3 is 0.86 dice

So, using 2d to 3d as an example

2.0 to 2.2 dice = 2d
2.3 to 2.5 dice = 2d+1
2.6 to 2.7 dice = 2d+2
2.8 to 2.9 dice = 3d-1 or 2d+3, your choice
3.0 dice = 3d

that means there are "dead" regions, but you might not care.