I am currently with bluewho. They provide plans where the fraction bandwidth/quota = 15/1. The plans I am reselling, start from 25Mb quota and go up to 100Mb. How much bandwith should I assign to each of them in order for the websites to function properly (not to run out of bandwidth).
When I ran my hosting business, I always stuck to 20:1 e.g. 250MB/10GB 1GB/40GB, but it depends what the account is used for. If you're going to have lots of large files backed up, but will only download/upload once or twice a month, you may only need to offer 10:1 or even 5:1.