Question: How to correctly estimate bandwidth requirements?

Hi
I am new to the game, have done some websites and decided to get into the hosting business as well,
so many reseller plans out there and it seems that it has been made quite easy for us little entreprenours.
Everything is going fine, but I was wondering if anyone could give me tips on how to properly estimate bandwidth requirements
for a website when offering hosting. I mean as a host you sell packages with limits to almost everything you give right, so how to correctly estimate bandwidth? Does anyone have any rules of thumb or other advice that could be helpful.
Many thanks to anyone who responds and also to all of you in the community as these forums are really a great help.
Sami

I'll give plans have b/w = diskspace allowed *(25~100) depending on your plan options, and charge for extra $ for additional b/w required.
Why u want to estimate b/w for users? Isn't that the users will determine how much they needed?