Your foot's instep is one of the things that controls this. A very narrow trouser leg will hit the higher part of the instep. A wider bottom will reach the lower part of the instep. Its your choice.
Also, the upper part of the trouser should be altered before the length is adjusted. Then being fitted the belt must be on and cinched up to the degree that you normally wear it. If you dont have two different leg lengths the fitter will mark only one leg.
For the person that has different leg lengths, the fitter will mark one leg. Then he will take a short stick, place one end on the floor behind one leg and mark the leg at the top of the stick, and repeat it on the other leg. when the trousers are on the cutting table being marked for cuffs
the two marks will show the different leg lengths. If you do not inform the fitter that you need two different lengths, he will assume that both legs are the same. We all know about the low shoulder and high hip. That high hip is going to be your longer leg.