Forgive me if this is hush hush info but I'm doing some research on how much space and bandwidth the average joe bloggs uses.
Can you tell me what percentage of users use x amount of disk space?

I.e.

30% of users use < 50 mb
40% of users use 50 mb > 100mb
20% of users use 100 mb > 250 mb
10% of users use 250 mb > 500 mb

etc

To answer your next question, it is to calculate how much overselling is actually viable. I'm a true believer that no host is 100% not overselling. I am not planning on being a high overseller, but I do want to see if I have around 10% to 20% more resources to sell than I've paid for.

Thanks again.

90% of users below 100Mb

Its not only the amount of diskspace that must take into consideration as things such as server load also plays a part.

If you oversell too much and the load gets out of hand, you might lose the entire machine even when you still have diskspace.

96% of my users are below 100 MB
96% of my users are below 150 MB
100% of my users are below 200 MB

I would be cautious about overselling. I personally prefer to not oversell more than 33% of the client's space, but I can afford to do so. Some may not.

To answer your next question, it is to calculate how much overselling is actually viable. I'm a true believer that no host is 100% not overselling. I am not planning on being a high overseller, but I do want to see if I have around 10% to 20% more resources to sell than I've paid for.
That is a reasonable amount to oversell, we oversell on shared hosting packages marginally but never reseller or VPS products. As a guide, MOST of our customers use about ~32mb for every 500mb that they purchase, but it is not uncommon for some to use substancially more/less.