This is frequently how jobs get quoted in our shop, for the reasons Strathead has mentioned. However, some semblance of common sense can be applied.
Each circuit on a booster power supply will be listed to 1.5, 2.5, or 3 amps (DC), typically. The power supply itself will have a max draw of 6, 8, or 10 amps (DC), typically. The power supply will have 4 or 5 output circuits, typically. As it happens, if you load up 3 or 4 of the circuits to their individual amp draw, you can't use the 4th or 5th circuit. So, if you look at the existing power supply, and it's using only 1 or 2 circuits, even if you assume they are maxed out, you can still add another circuit or even 2 as long as you keep the draw under the panel maximum. This all requires that whoever is quoting the work actually get out of the car and put their eyes on the inside of the booster panel, or have the customer send good pictures of the guts.
The other option is to meter the max amp draw on the installed circuits and go from there, although there is usually push-back from the building occupants about ringing the horn/strobes.