Luke (I am your father - Lollz, sorry I just couldn't resist),
as the others have mentioned, length of the handle foundation is usually (at leas as far as I know), what is used to determine the actual length of the handle.
Now you can cheat a lot with the placement of the ring knot and fool the eye into believing it's actually a longer or shorter handle than it really is. For example, I've got an 8 foot bullwhip in the works and it has a 10" spring steel handle (so a slightly smaller foundation than on an Indy for example), but I wanted to make this a very versatile whip, so I fudged (or rather will) the knot placement, by having it end a bit before the 10" mark. So it will look somewhere between an Indy and a longer handle bullwhip... Inversely, if I was making a Raiders bullwhip, I could either use an 8" steel spike (a big nail) and place the ring not at the end of the handle, so after the end of the nail, over the thong part of the whip,
OR
I could use an 8 1/2" or even a 9" spike, and place the knot over the nail, so as not to place the knot over what is generally viewed as a transition zone from handle to thong, which is, by definition, flexible to a greater or lesser degree and so is not really the best place to make a knot; it's always a better idea to place knots over solid areas that do not move.
Keep in mind that the ring knots on bullwhips serve no function beyond that of hiding the transition from the "standard" checkerboard pattern used over the handle, and the herringbone plait used on the thong part of the whip. That's it really, that's it's reason for being. Yes, it's true that on a bullwhip with no added binding, the thread (and sometimes leather), used to make the foundation for the ring knot, will add a bit more stiffening to further strengthen the area over which the knot is tied, but I personally don't see the point of that approach. I feel it a better one to use a longer spike if you want a longer looking handle and to tie the ring knot over the actual spike. That way you are not taking any chances that the knot will eventually come undone because of continual bending of the thong. But that's just my opinion
).
Take care and hope this helps,
Franco