At the end of the day I feel like you are correct in practice but the advice to follow mathematical relationships is still solid. Yes you are probably going to

At the end of the day I feel like you are correct in practice but the advice to follow mathematical relationships is still solid. Yes you are probably going to need to deviate from the mathematical relationship at some points, but these are generally helpful to observe and understand the design challenge. For one thing have a mathematical basis acts as a good starting point. This is helpful for more junior designers, it is good when the assets that are in flux and it is generally useful for getting things close before tuning. Second, having some mathematical conception of the layout can help provide deeper insights into the problems. You can start to formulate things in more regular terms and you can start thinking beyond a layout configuration and consider how the problems generalize. Ultimately people have different methods and you should do what works for you, but I think that this point is still good advice in general.