Focal Length Question for PCX Lens Mod

I'm "attempting" a mod on a XP-G R5 powered Light.

At Anchor Optics, they offer several "Focal Length" for my 16-16.5mm Dia Lens.

I don't know how to determine the correct focal length for a PCX type lens.

460" or 11.6mm is the distance between the lens seating surface and the top of the die.

Is that distance the Focal Length ?

Also, under comments, the term "CTD" on some of them, any idea what that means ?

http://www.anchoroptics.com/catalog/product.cfm?id=252&s=diameter&d=a

IF Anchor doesn't have one to fill the bill exactly, I'm gonna glue a outward spacer and get one at Kaidoman.
http://kaidomain.com/ProductDetails.aspx?ProductId=9111

edit to add, Surplus Shed, nice spot with optics too.
http://www.surplusshed.com/finder.cfm

Thanks

If the lens were focused on your LED's die perfectly, then the measure of the distance between the lens seating and the actual die would be the Back Focal Length. (Please someone correct me if I'm wrong) The actual Focal Length, you can imagine being measured from somewhere inside the optic.
There is an old thread around here somewhere that I'll have to dig up, but it has a formula for determining the optimal focal length for an aspheric lens, based on the lens diameter and the prominent beam angle. If I remember correctly, it usually comes out to around D = 1.4 * F.L. Unfortunately, none of those optics I saw on Anchor Optics have a short enough F.L. to help you with throw really.
As for the "CTD", I'm not sure what that denotes.
Good luck with your search, DrVette. You might try Edmunds, Thorlabs, and Melles Griot as well. They have big selections of optics.