Usually when a contractor does spraying he is told what rate of chemicals to use from (an agronomist's) recommendation. The contractor doesn't decide for himself how much he is going to put on.
So the contractor goes on said courses in order to be able to legally spray, then in your opinion he does not need to know how to calculate the dose rates which is written on the label, whereupon he simply believes what Agronomist states without checking the math to ensure no mistake in the calculations, then proceeds to spray willy nilly
If a contractor cannot work out dose rates - he should not be doing the job imho, period!