How is the ampere percentage of a circuit breaker selected for different loads
There is no formula, not everything in EE is described in formulas. If it was EE would be no fun. If you like formulas, go study maths instead.

The percentages are based on experience.

Some devices, like motors and welders can draw a huge current when they start up / you switch them on.

Some do not, these are the resistive loads like heaters, ovens and toasters. These draw the current that you would expect from their rating, not more.

So that is why depending on the expected load, a certain trip current is selected.