Fees charged by strata management company

Our unit block comprises 18 units and has been using the same strata management company for years. 

We are reasonably happy with the service but the accounts always show quite a variety of charges which I am not convinced are all justified: for electronic storage, other storage fees, work orders, admin expenses, quarterly report fees, schedule b & d fees, sundries, as well as standard management fees.

Is there some rule of thumb for evaluating what fees are reasonable? The building is in good order in general and the issues handled are low level (e.g. such as organising a visit by a tradesman maybe once a month).