Are you using 1.8 of Factorlib??
not sure about previous ones, but in 1.8 you can right click on the preset (Cmd + Click?) on preset and get up a graphical impression of the knobs on your screen and seemingly you can tweak away in the way you are talking about.
As I say it might not have been in the previous Factorlib, if it was I missed it too and only noticed it the other day when I was trying 1.8 out.
give it a go.