How to determine the cutoff of hypermutation?

Hi. I know that the TMB of 10 mut/Mb is usually used as the cutoff of hypermutation. But I vaguely remember that I have read a paper, which determined the cutoff of hypermutation using a algorithm (a+1.5×b???) and finally 9.44 (probably) was used. Does anyone have any clue about this?
Thank you.