The current “Jump To Value” option will do exactly that when active or only react if the user touches near the current value.
I would prefer a “Relative” mode when “Jump To Value” isn’t active. An extra mode is also a possibility.
TouchOSC has “Relative” and its documented as:
“In Relative response mode the initial touch will be remembered and not trigger any change and subsequent movements will be applied relative to the initial touch position.”