How to Calculate Correct Fuse Rating and Tolerance for Home or Device Circuits?

Hi, I have a question as simply as possible or how is it calculated which fuse to put under a given device or at home. I mean specifically what is the formula for calculating this and what tolerance should be adopted. THANKS

Fuses are of typical sizes, e.g. mains fuses are:
2, 4, 6, 8, 10, 12, 16, 20, 25, 32, (35), 64 etc ... there are no intermediaries.
Each device that you want to protect has parameters such as current consumption or power on the rating plate. As far as it is a resistive device, the matter is simple P = UxI, where I = P / U.
In the case of reactive receivers, the matter becomes more complicated, but in these cases the manufacturer specifies the current consumption.
You select the fuse redundantly (it is supposed to work when the rated current is exceeded). So if the device uses, for example, 10A, the most appropriate fuse is the higher row, i.e. 12A, etc ...
A separate issue is the types of beapieczniks (quick, delayed ...) but that's a lot of literature.

Well, maybe a little explanation about these formulas what is what

As for the mains fuses (excluding radio fuses Sp-7),
these are not what the predecessor writes about.
E.g; 2A, 4A, 6A, 10A, 16A, 20A, 25A, - small sockets
32A, 35A, 40A, 50A, 63A - large sockets.
there are also so-called BMs, power fuses.
I think the issue is mainly about powering electrical appliances
such as motors etc.
OHMA law I = U / R, ------- I, is the current
U, that's the voltage
R is the resistance
P = UxI ---- apparent power V / A
P = UxIxcos.? ----- active power ---- Wat.
P = UxIxsin.? ------- reactive power ---- War.
You can calculate the value of collateral knowing the so-called K-factor, depending on the load, e.g. of the engine.