Utility Meter Billing Formula

Hey everyone,

I am stuck. I need a formula that will total usage, calculate allowed usage, and calculate over usage. Would it be a COUNTIF formula?

Example 1:

Beginning Read D1 - 2000
Ending Read D2 - 2400
Total D3 - 400

Allowed Usage D4 - 400*.05 (it takes total in D3, counts up to 500, but never exceeds)
Over Usage D5 - 0*.08 (it takes total in D3, counts anything over 500)


Example 2:

Beginning Read D1 - 2000
Ending Read D2 - 3500
Total D3 - 1500

Allowed Usage D4 - 500*.05 (it takes total in D3, counts up to 500, but never exceeds)
Over Usage D5 - 1000*.08 (it takes total in D3, counts anything over 500)