Is there a reasonable rule of thumb for determining how to allocate transfer and storage space limits when determining what hosting packages to offer?

Not really, to know how much data transfer your site will require you need to figure the amount of page views you will receive and the average amount of page size.

Our average customer uses about 1/8 of the transfer offered with their plan.

Count on 90% of your resources being used by 10% of your customers...

Most people use a 1:10 or 1:20 bandwidth, space ratio. For example 50MB space should be 500MB bandwidth would be 1:10 and 50MB space and 1GB transfer would be 1:20. Hope this helps. :)