If the project is being benchmarked as in concept planning, then applying an operating expense ratio of between 45 percent and 50 percent against gross sales would be appropriate.
If pro forma financial projections are being prepared, then operating expenses need broken down into its component parts.
As Earl mentioned, variable unit cost (VUC) can vary greatly.
For example, benchmark VUC is $1.85 whereas I just finished a project where the client’s VUC was $1.86.
This operator is adding hot wax product. The product has cost of goods of $1.00. So, VUC would be $2.86 for each service sold.
Conversely, if unlimited washing is planned, then sales volumes need broken down into its component parts.
For example unlimited subscribers typically wash between 4 and 5 times a month whereas the average customer might visit once a month.
Hope this helps.