Sounds like you are welding a tee fillet joint. Setting the amperage about the same or 5-10 amps higher should be fine. The thicker material will absorb more heat than the thinner material, so favor the thicker side to keep from blowing through the thin side. By the time you complete one side of the joint the material temperature will be hotter, so less amperage will be needed on the other side of the joint or you can allow it to cool before finishing the weld. Using a foot control for the amperage is the best way to control heat input for GTAW in thin materials such as these. If you see the puddle getting out of control, just back off on the pedal to lower the amps (just like the gas pedal in a car!).
Hope this helps,
Marty