It's everywhere. You have to find the worst place(s). You can't say it's good and then have someone walk around the tank and say it's worse over here. If you are not there to eyeball it and are just giving guidance to a crew that is on site, it could get tedious and expensive. For 10.5.2.1 you could start with measuring at the middle of each plate and at the plate vertical joints, combined with an elevation survey of the first round seam to establish tank lean (which contributes to plumbness). For 10.5.2.2 you need to examine each shell plate individually. For 10.5.2.3 depending on the tank size perhaps starting with 30 feet spacing. If I have the budget I just get a 3D scan of the entire tank, roof, floor, etc from the inside.