Knoopenzo.nl (Knots etc.) is a Dutch website run by Theo Slijkerman, a member of the International Guild of Knot Tyers. The website has a number of very interesting knotting resources, including calculators for working out the amount of rope needed for a rope mat, a turks head and a braided ring.
For pioneering, his square lashing calculator is pretty handy. Given the diameter of both poles (Dp and Db), and the thickness of the lashing rope (Dt), it will calculate the length of rope needed. The full calculator is available here, and a simplified table showing the lengths for a 10mm rope is here, in English and Dutch.