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Hi, I'mike G and I'll be assisting you. Yes you may, not a code violation.
There isn't an article preventing it.
There isn't a code article. A change in size of the main to a lower rating on a buss rated higher is not an issue. No code reference
I know the type. He should cite the code if there is a violation
3℅ on your wiring 5℅ total yours and the utility combined.
Your calculation is for 3℅. If you test for voltage a 5% drop is allowed for the drop from the utility.