A campaign centered near the Strip is buying visitors as well as residents
Geography here is not a radius problem, it is a population problem, because two very different audiences occupy the same map.
A practice in Paradise or near the Strip corridor sits inside an area with an enormous transient population. Draw a standard ring around that address and you are bidding for hotel guests, convention attendees and people who searched from a plane seat, alongside the residents you actually want.
Sometimes you want both. A same day emergency visit from a visitor is real production. But you want to know which one you are paying for, and default targeting hides that from you entirely.
Build resident targeting around where your patients live and drive from: Spring Valley, Enterprise, Summerlin, Green Valley, whichever of them actually show up in your patient file. Exclude the outlying towns you will not realistically serve, Boulder City and further out, before you spend a dollar there.
Drive time beats mileage as a boundary. The valley moves reasonably well most hours, but a patient on the far side of the 215 loop from you is a different proposition than one four exits away.