Available hours calculation in Utilization!!!

Hi Brains,

I need to work out utilization of particular department in our company.
Normally we get a report with the following informations from the system :

Actual hours spent by an resource :XXX
Hours need to be billed :xxx

But i need another column ->Available hours ( calculation based )

Utilization formula would be billed hours / Available hours * 100

Available hours

Availalble hours workings should be as follows :

Resource who works on onsite - 8 hours per Day
Resource who works on offshore - 8.75 hours per Day

A resource may work on onsite for few days or he/she may be full month onsite ( or ) fully offshore.

Working should be hours based & project based

Attached worksheet for further understanding.

Eagerly waiting for reply