Maybe fission and fusion could be used to directly impact the quality of reagents. I don't see the values as being linear except for fission.
I could see two mounds of reagent being randomly cut in half and thereby reducing both to the usability of half the quality. The math would not be colinear between interactions but the advanced skills using reagents makeup for the losses.
Fusion should function under some form of quality control that would take out parts of the less viable portions and make the best average mix without diminishing the pile too much. Maybe the better pinch could be the base and the second could be point value divided by the better and then modified with some extra constant.
Fusion (example)
Pinch_1 value = 6
Pinch_2 value =8
Pinch_2 is the base @ 8
Pinch_1 is the working value @6
working value/base = 6/8 = 3/4
set_constant = 2
Formula
Base + (working value * set_constant) = potential finish
8 + (3/4*2)=9+1/2 ~ 10
Let it round to nearest integer and a new reagent pinch is made.
The constant can be just about anything that would allow for modification changes, best case the set_constant would be the part of the equation that would be modified by skill (say every 150 or so bonus the value goes up an integer).