I would try using opks. They might help you better detect you ovulation since your cycles are not being consistent (welcome to my life). I just started using them today, so I don't know much about em. I would just calculate the average of the 28 cycle and 31 cycle like you have done and bd around those times. I would even start on cd 10 through cd 18 just to cover all the the bases.

Best of luck and wish you nothing but
