There is no universal number as far as what each corner should weigh. As power goes up weight generally needs to be moved. Big power can require the car to be neutral or weight being moved to the left rear. Scaling allows you to establish a baseline. The car will tell you what it wants from there.

If you have a car you're familiar with or a personal car, scaling the car after it's been sorted, just as it sets with no driver, and recording the information, will save time. It will allow you to put the car right back after having something apart or replacing parts without having to load ballast in the drivers seat. Typically I record with and without numbers. Unless the usual driver gains or loses a bunch of weight, it works out fine.