I think this is common with the wooden frame.
I made my table as per the book, it sagged in the middle i checked with the longest straightest edge I had. I added some legs to the middle of the span and tweeked until there was little or no gap under my straight edge.
Is used a long spirit level acros the width at various points to check it was consistent and not warped.
I'm now happy its good for a home build, I guess its accurate to 1-2mm.
A good check is when you tack the uprights to the base frame and lay the top tubes on do they all meet up reasonable well when cut to book dimensions and meet up with the seat back without too much tweeking.
Again I check all the top tube with my spirit level until I'm happy.
I had to take about 5 boards off the rack at B&Q until I found one that looked flat, that got me some odd looks and a lot of heaving! Think I bought the planed timber that was tightly bound into a stack of 4 or 6 that showed no obvious signs of splaying out between the pieces of timber.
Hope you sort it
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