While I was on the plane back to the UK, I did another investigation of the slipstream code (it was a 14 hour flight...)
It turns out that original GPL, 67X and 68 use a different formula to the other mods and the difference is bigger than I expected. It also means I made a slight miscalculation in my 2.1b patch. I will remove it and make a new one in two weeks
FYI I have almost completed making the new version of the patch. I am just doing some final testing of server security etc. for online play.
The update modifies all mods to use the effect in blue, rather than the one in orange. I did not notice there was a difference until I analysed the code again on the plane home to the UK.