For the typical tapered, semi-eliptical small glider planform, washout of the wingtips should improve performance by reducing lift induced drag. For thin wing wood like 1/32" or thinner, heating the area with your breath and bending works fine. Typically, for this size glider the area bent up is the last 1" of the trailing edge nearest to each wing tip with the bend starting about 1/4" from the trailing edge in the chordwise direction as measured at the very tip. I have found that for the Stan Buddenbohm Littl Sweep about 1/32" of washout on each tip works pretty well. Larger, higher aspect ratio gliders with very narrow tip chord,like the Ron Whittman Super Sweep (hand launch glider), use as much as 1/8" washout bent in. For thicker winged gliders like the Littl Sweep Category III/IV glider, washout can be sanded in. Hope this helps. Also, if the washout is not identical on both wing tips, even if it is only 1/64" different, it can cause the glide circle to change. Washout for very large gliders is sometimes progressively added to the trailing edge in progressively increasing amounts over the entire length of each wing (from center to tip).
Hm. I added some washout using my breath and fingers, but it turned out to be way more than 1/32 inch. It was also uneven, my glider started doing weird things in the flight circle like you said. I ended up just flattening it out to normal.
Is it just my building skills, or is it possible to add washout accurately using the breath/finger technique? I just thought of something that might work, although it might not. What if I put each wing tip on a flat surface, and inserted a 1/32 thick sheet of wood under the back edge?