Funny you mention that "rule". It's not very accurate, and becomes less accurate the higher the inflation/interest rate goes.

I just worked it out; at 80% inflation it would take 1.18 months to double, not 0.9 like the rule of 72 would suggest. That's a fair difference, and certainly more in line with intuitive expectation.

Not that the OP isn't being overly finicky. Both round to 1, and only one significant digit was given anyway. But still.