Originally Posted by Zen Scanner
Seems to me that given specific quantity needs, the correct answer is that this is not an appropriate time to use estimation.

Or, an appropriate rule of thumb would be: given specific quantity needs, always round up, even if it's further to round up than down. Then, if you come up short, you may have a problem, and should do a more precise calculation. If you don't come up short, you know you're okay.

Somehow I doubt that's what the question-writer had in mind.