When measuring taper, different tubing and casing forms require specific size contact points due to the engagement of the point on the thread. Plus, API 5B mandates the proper size.
8 & 10 Round: The proper size causes the point to contact the thread flanks at the pitch line, effectively, measuring the taper on the pitch cone.
Buttress: The selected size makes contact on the root of the thread, ensuring measurement at the minor diameter for pins and major diameter for boxes.