Gordan, the chord legnth includes the flap chord in your figures. For instance, 10 inches from leading edge to flap hingeline, plus, oh, say 3 inches flap chord at the root, equals 13 inches.
So, 20% root chord times legnth, 13 inches, equals 2.6 inches thick at the root. 18% would equal 2.47 inches thick. 16% would equal 2.08 inches thick.
Ty is correct about the entry radii of an airfoil. The blunt NACA 4 diget has the greater radii, but it presents a higher potential for induced drag.
Going to extremes the other way, a sharp pointy airfoil, think of a 1/4 sq set so the point is at the extreme leading edge. Such an airfoil may penetrate turbulence, and groove in level flight, but as soon as the airfoil is rotated so that the angle of attack changes to creat the lift needed to negotiate a turn, the airflow seperates, and the wing begins to enter into a stall.
So often times somewhere between these two examples lies the ideal airfoil for the job at hand.
I've had good success with a particular airfoil known as NAC 63A modified. It is as thick as the 4 digit airfoil, but the entry radius is smaller, while still not being to the point that it stalls as the pointed airfoil does. It penetrates better in wind and turbulence, grooves well, and still doesn't enter into stall territory. at the speeds and angle of attacks we use.
It may be said that the most important part of our airfoils is the entry profile. The part from the leading edge to possibly just aft of the high point. All that's behind it, many believe, does almost nothing for us at our reynolds numbers.
I'm not sure this is always the case, as many feel that the Pathfinder airfoil has some positive characteristics that are attributed to it's particular airfoil. Indeed, many combat ships and several full scale aerobatic designs use about the same airfoil.
Like any exercise, when designing a stunt plane, we need to make some basic choices, and then use what works best around those choices.