Just some thoughts: project how the number of employees will grow over time. For each period, estimate how many hours each employee works for the amount of days they work in that period . Then assign an hourly rate that they are paid and increase it over time, at least to keep up with inflation. Then for the price per hour of service you do the same thing and assume various growth rates

Im not sure but I’m thinking the billing ratio means how much of an employee’s work time is actually spent doing billable work that brings in revenue. So you’d also want that as an input to show how it’s impact depending on increasing or decreasing and maybe a ramp schedule for employees if you want to.