My simple thoughts follow:
I start from the proposition that Fixed costs are what it would cost you to have a legal aircraft in the hangar, even if it was never flown. That figure is then divided by the number of Group members and again by 12 for individual monthly rate. Groups I've been in will round this up to the nearest £5 or £10 to cover contingencies.
Includes: Insurance; Engine fund*; hangarage; radio licence fee; CAA fees (e.g. CofA , divided appropriately) etc.
Hourly rate is then whatever it actually costs to be come airborne - fuel; oil; realistic proportion of estimated check costs & fee etc. Includes an estimate of consumables (oil, plugs etc). This list and the one above are not comprehensive.
This philosophy attempts to spread costs fairly, so that if yoou have a Group member who never flies, (s)he still pays for upkeep of ther asset. Those who fly a lot pay a commensurate amount to reflect the wear and tear they put onto the aircraft.
Hopefully that helps.
* You may wish to include this - or a proportion of it - in the hourly rate if engine is lifed.