Autotransformer Starting

I am designing medium voltage motor starter. rated voltage for that motor is 4160 Volt.

but the problem is, is there any calculation to determine, how many power (HP) of that motor shall be using Autotransformer Starter, and how many power of that motor shall not be using Autotransformer Starter ?

because in my client project's spect state that medium voltage motor shall be using Autotransformer Starter. I see "some strange" in that specification. I have planning to submit some suggestion or revision to my client. but i dont have any strong data to motivate my planning.

i am very expecting your help

thank you.