Sometimes in app notes i see something like @assume that magnetizing current value is 10% of the primary current". Why? How to choose the magnetizing current value the right way?
Magnetising current will be practically irrelevant in a good mains transformer design. As it's in quadrature to the load current, a mag current of 10% of the full load will increase the full load current of the transformer by 1% (Pythagorus). 10% is a very conservative estimate, it will typically be smaller.
Magnetising current is sensitive to the transformer inductance. Transformer iron varies in relative permeability from a few thousand to many thousands, depending on price, and strain history. If a transformer is built from Es and Is, or two C cores, variations in size of the inevitable airgap could cause a 10:1 change in inductance, together with a similar change in magnetising current. So magnetising current is measured on a transformer as an assembly quality test.
There are mains frequency transformers which have a different typical ratio of magnetising current to full load. Current ratio transformers will use a special high permeability core to reduce mag current even further, as it represents an error term in the current ratio. Microwave oven transformers will have a mag current in the same order as the full load current, perhaps only 1/3 or even 1/2, as they are built down to a price, run hard, and fan cooled.
Ferrite core transformers will tend to have higher relative magnetising current than iron transformers, due to the lower permeability.