I tend to go with Robins suggestion of a higher monthly cost, we have our (PFA type) on a £85/month between 5, this covers all the fixed costs including (hopefully a bit more than) that needed for maintainance and engine fund (even if not lifed you will still get to the stage it needs replacing). all we then need to do is put fuel in (about£25/hr) and pay the landing fees. With a non PFA type you probably do need to have an hourly rate to cover the intermediate checks.
