Thank you geoffp! I really enjoyed doing your kata.

Dear geoffp, first of all let me thank you for this extremely interesting kata. It was such a challenge for me as a hobby programmer to try and solve it. I think I finally devised a solution, but still I'm getting KO from the "random seed" test. I manually tested my program by (painstakingly!) remaking all the calculations with a spreadsheet and cannot find the bug. Maybe I can share my code and some details in private to you to get some advice? For example, in a random test my program yields 20.1813457339 vs expected 20.1270180255. I went through the computation manually and got 20.1813810942805. Maybe I'm simply replicating the same concept error in both my program and my spreadsheet :) That's why I need some advice.
Whatever your reply, thanks, thanks thanks! Great kata.