Have you considered the minimum fee approach as not just a monetary figure but setting a maximum percentage change from the existing fee structure?

We are currently researching the potential to introduce a minimum fee for some audits. This includes the possible impact on those bodies which had a current scale fee below any new minimum fee structure. We are engaging with all funding bodies regarding the fee procurement so that they are aware of our minimum fee proposals. We are trying to get to a fair scale fee for each audit so that all participating bodies make a fair contribution to the cost of the audit contracts.