This means that this tool has been designed to provide you with an idea of a minimum hourly rate to charge for non-commercial creative services.
Factoring in elements of cost plus based pricing and years of experience, this is a basic, standardised pricing tool to serve as a guide.
If you have total expenses lower than £1000 each month - insert £1000 in the monthly expenses section. This will serve as the minimum in order for this formula.
The reason for this is because part of this formula relies on cost plus pricing methods, so if your expenses are significantly lower than this amount, this may reduce the rate to below minimum wage (and we don't want that!)