There's a few half-life threads already. Slow_Mobius pointed out already that just multiplying half-life by five is a good rule for how long it takes to clear a drug from your system.
You'll get different answers because there's different interpretations of when you can really say a drug is out of your system (at 95%? 99%? etc.).
What exactly were you trying to find?
Hey, I resent that a little. I can see scaring teens by making them fill-in spreadsheets, but dammit, I miss getting paid to do that.
That is a big range, do you have a link to the calculators?
That is interesting: the first one just uses 24/37 as a multiplier, the second one 24/21.95 as an exponent.
Anyway, Friday is always a day for sushi and movies, I say go for it.