This distance between a girls legs directly below her pubis symphsis.
(There is a slight distance decrease as the inner thigh usually bulges in, about 1-1.5" down from the perineal. This is natural in even the most perfect 18 year old specimens)
Tell your woman to get in her bikini. Instruct her to stand in an upright and standing position, as if she was waiting in line at the grocery store with food that she will cook for you later tonight.
With her facing directly in front of you, make sure that her ankles are touching each other.
From her ankles to her crouch, the inside of her legs should never touch. Ever. This is the GAP TEST. If her legs do touch, you will need to consider taking drastic measures to remedy the situation.
Calling her fat will not help, add -ass to the end of it, that usually increases response time.
Also see The Bridge
and The Valley
to learn other important female attributes.