How flat is flat enough for a building board?, considering that balsa swell and shrink as the humidity content in the air changes? I'm building on a 6 mm thick laminated glass sheet on a 50 mm thick (kitchen top sheet from hardware store) for many years. Never had problems with skew wings or fuses.
You build a plane on a laser levelled board, but by the time you come to cover it, the wood may have swollen or shrinked abit due to moisture in air. Then, when you cover the plane and shrink it eneven, you have a skew plane. Their are tolerances we have to accept and something like a length 50 mm kitchen top is a good,stable base.

Cheers

Bundu