I'd suggest that when you buy, you build in a margin on top for unexpected bills occurring in the first year or so.
So for a £45k aircraft, with 4 people take a figure of around £50-£55k.
Then, look at the cost of insurance, hangarage, 6-month check and annual and divide it by the number of members, then that becomes your monthly, in case it never flies for any reason.
For your hourly rate take the fuel flow (be pessimistic). Look at the outstanding life of the engine to 2000hrs and divide the replacement cost+fitting by the remaining hours to work out the contribution for the engine fund. Again, be pessimistic - it may not last that long.
The problem is that there is a balance to be struck between paying a lot in your monthly and a cheap hourly rate, or the other way round.
In my view you have to ensure income whether or not the machine flies, and so I tend towards a higher monthly. If you only fly 1 hour a month, it will be quite expensive, but it becomes cheaper per hour the more you fly.
Oh, and you will find that when you ask your other members how much they intend to fly, assume they will do around 75% of their intended figure.
Early on you want to stuff as much away as possible for the rainy day. Especially as we have the CAA (Campaign against Aviation) imposing new charges on us, such as the new Part M maintenance requirements, Mode S and ELTs.
But it is worth it.....