I think bikes are designed to fit into a range for the majority of riders so a fit on an existing bike will always be some amount of tweaking. You already have the bike, the geometry isn’t changing.
Ideally there is a goal of getting a good balance of weight that also puts the rider in a comfortable riding position. Anyone who has ridden a crank forward bike can quickly understand the effects of poor weight distribution. The rider to bike weight ratio is so significant that where it is located impacts how a bike reacts.
The curveball is when a rider experiences pain, or has physical limitations, especially with age. I’m not a fitter, but making on the fly adjustments while on a ride should be part of it. Teaching someone the how and why, even if is not absolute, will be valuable an hour or two into a ride. Obviously swapping a crank mid-century is not a viable option.
John