ThxThe stop charge covers the cost of your overhead and travel to the site--for instance $25.(Rent, insurance, truck payment...)
Then about $2.60 for each 1000 sqft. This assumes a labor cost of a dollar per thousand sqft, and average chemicals cost of $1.60.
The labor cost includes the cost of the machine time--which is the cost or payments on the equipment so that you can repair, gas it, and replace the equipment when it wears out.