Well, I'm hoping that Ollie will jump in here and give you a more definitive answer, but I do know this..

In my hobby experience, I've found that most people preach a simple axiom that "thicker = more lift" and "thinner = less drag". Under certain conditions, this is true, and if you've stuck to similar planes for a while, you can believe that it's that simple. (Of course wing loading, aspect ratio, etc., all have an effect too.)

But there is a point, related to size (or "scale") where that breaks down... roughly when the wing chord goes below 5 or 6 inches, and at that point, a flat plate is nearly as efficient as a traditional airfoil. (You didn't mention the size of your Giles wing.) Of course, there are other things you can do to prevent tip stall too, like adding a bit of washout at the tips, which I'm sure you know, or changing to an airfoil with a higher critical angle of attack for the airspeeds in question.

As for the relationship between root and tip thicknesses... Help, Ollie!