Well, the base would just be considered within the vicinity. There isn't really any set line. The definition of it isn't necessarily the important part.
Once it has been determined there are no hogs available, you are allowed to open fire on someone that is walking with you. This could be at the base, leaving the base, or in the middle of the map. Just be sure you are moving to your navs, and not chasing them - and you would be well within the rules. As long as your first priorities are to get in a hog and race - you shouldn't have too many problems.