The starting point for deciding on the level for reserves is to take 50% of the annual precept. Then it is prudent to add some specific provisions for things that might go wrong but would not be covered by insurance. Finally, it is reasonable to add any specific capital projects that are intended to be paid for out of the reserve.
If the reserve is a lot different from 50% of the precept, the auditor will expect it to be explained, but will accept a reasonable explanation along the lines outlined above.