Remaining Value Formula

I think you would need a child table here to assign equipment to a client with a start, term, end date and assignment duration. This way you can track the total amount of time it has been used on many client assignments and not count the time it was not assigned to a client. the formula field would include a summary of the duration months on assignment across multiple assignments. Something like: ([summary field]/36)*[value]

Let me know your thoughts.