My understanding is that empirically, Haldane found the limit was a multiplicative factor of ambient pressure. I think originally about 2x, later backed off a bit, but still some factor of ambient. With ambient pressure on the horizontal axis, the M-Value line would always increase (for positive multiplicative factor).
See e.g. "Robert Workman" at Dive-Tech: Decompression theory - Robert Workman and Prof A Buhlmann (off the top of google's list)