RE: How flat is "flat" for a building board?
As a practicing fussy person, I am humbled by the level of obsessive behavior reported here. I used to think that if my structure looked strait, then I was OK with it. I would look at it and check it with benchmarks and all that good stuff. Then I would cover it and check it again. Then I would go to fly it and come home and check it again. When I got all through the good news is that I had a straight plane and hadn't used 6 ounces of fuel in a season. BIG savings in costs.
Do the job well and then go out and fly it. Otherwise you can plan to use the plane as a display item and apply to the Guiness Book of World Records for an honorable mention in the World's Fussiest category.