The easiest way to add washout is to flex the tip and use a heat gun to take out the wrinkles (it takes about 1/2 hour and two people)
Put a meter on the wing and get a baseline first. Flex the tip - trailing edge up, leading edge down and shrink out the wrinkles in the covering. Continue flexing the tip until the covering cools. Now repeat on the bottom of the wing. Each time you do it you'll find that the wing has "given" a little in the right direction. It usually takes 5 or 6 times on each side.
1/2 - 1 degree of washout is plently. Just matching the incidence between the two tips will probably make a noticeble difference.

Chris