Give a man answer and he will work for a day. Give a man an equation and he will work as long as he likes.
A squared + B squared = C squared
works on all right triangles. invaluable for anyone who wishes to build anything.
where c represents the length of the hypotenuse, and a and b represent the lengths of the other two sides.
I would measure the total distances involved (A and B) and do the math with those numbers, rather than trying to tack on the figure calculated for 'extra' length.