if you are using the OEM idle valve, it also has this specific reaction(from what i have gathered by other users): if set below 29% DC, it tends to reopen and if set even lower, it opens more. So the fully closed position of the valve is around 30% DC give or take.Also your DC curve is all over the place..
as mattias advised, this is the ideal method of setting up idle and i think it should be made a sticky!