Calculate field depending on Current month

Hi!

I would like to fill a field (through workflow or creating it as a calculated field) depending on if it is the current month or last month etc.

The new field should work as follows:

I have a value field (Revenue) and a closing Date. So if the closing date is current month it should show 100% of the Revenue

If the closing date was last month, it should show 50% of the Revenue

If the closing date was 2 months ago, it should show 25% of the Revenue

If the closing date was 3 months ago, it should show 20% of the Revenue...

and no, it should not look at the actual number of days from closing date, but the actual month.

So if today is 1st of May and the closing date was the 30th of April, it should show 50% of the revenue since April is last month

regards,

Tomas