I am currently building a SSE and I agree with you that the wing will not end up straight.

I do not agree with the gentleman who told you that what you are building is "washout." If you follow the instructions as written you will be building "wash in." That is, the tips will be at a higher angle of attack than the root. This can lead to premature tip stalling. However, the SSE seems to fly fine with it. Most likely because the full span ailerons put negative camber into the wing tip when mounted to the wing. Wash in may even be desirable in an aerobatic model as it may lead to quicker snap rolls. Many electrons have been spent on this topic. Check out the following URL for the specifics.

http://www.angelfire.com/indie/aerostuff/washin.htm

I'm endeavoring to build my wing straight. To do that, I used a board that was 1" thick at the root and 0.86" thick at the tip. Check the math for yourself and see if you arrive at the same values.

Wishing you blue skies,