The gain trimmer will affect the slope starting at the cut-in point from my understanding. So where ever the starting point is, whether 12% or 40% or wherever, that point will start at 1:1. If the gain is left at 12 o'clock on the trimmer, the injection will follow the 1:1 slope. If the gain is turned down(to the left) the slope will pivot from the cut in point and then the slope will deviate and run shallower than the IDC slope.

Conversely, if the gain is turned up, the slope will rise, injecting more as the IDC gets higher.

http://howertonengineering.com/Aquamist_hfs3.html

Scroll to the bottom of the page and I have modified the HFS-6 trimmers worksheet for the HFS-3 control board. This should give you a visual representation of how the trimmers will affect the injection slope. You can plug numbers into the colored boxes at the top of the spreadsheet and watch the slope change. Please don't change any of the values on the other sheets as these are all the calculations to make it work.