They have two formulas (see SAMD10 datasheet, for example):
fSCL = 1 / tLOW + tHIGH + tRISE
fSCL = fGCLK / (2 * (5 + BAUD) + fGCLK * tRISE)
Am I the only one who thinks that the second formula make no sense? Specifically, the tRISE part of it. Unless they mean two completely different "fSCL" things in two cases.