40% is a rounding error? I hope not ;) (edit, fine 34%... but I'll say THAT might be a rounding error grin)
Second, the number it was being compared to in roenxi's comment was 9 million years, not 4 billion years.
So, yes, it is in fact a rounding error.
;-)
I think 100mya is perfectly fine. It doesn't undermine the point and this isn't trivia I plan to remember more accurately than that.