Criteria for medical necessity for any particular procedure would be a matter of payer policy - there usually isn't a standard guideline that would apply to all situations.
Here's a link to the LCD published by CGS for their Medicare jurisdiction - it specifies in the text that they consider hemodynamically significant stenosis to be "
generally >/50 percent of the vessel diameter":
L34062
Other payers may have different criteria - you'd have to search each payer's policy library to determine if they publish anything specific to these services.