free vac & car count formula

[BBE]

Is there a formula for how many free vac drops are needed for X volume your wash is doing?

 

[carwashireland]

Here is an interesting thread as I am adding free vacs to my iba's. I am space limited though so planned on having a time limit on the vacs. (Push a button for 4 x mins of free vac then button wont work for 3 x mins so the customer leaves and the next customer can use it)
Pay vacs are available also if more time is needed.