My own suggestion is that (a) of course Tanach was written before the discovery of irrational numbers, and that (b) the sea of Shlomo, like any real object, was not a perfect circle – it had width for starters, which gives you a choice of different places to measure from. In fact (though I don’t believe this to be true) you could derive a width of something like (10 Amos – 30 Amos / Pi) / 2 which is approximately 0.22 Amos.