So after careful consideration I decided not to go in the original direction.
The Cervelo supplied Saddle clamp accessory mount pictured above place the retaining bolt directly inline with the saddle rails.
I was unable to install, remove or tighten the bolt without moving the saddle fully rearward.
Given that this additional (third) water bottle cage is only required from time to time I mounted it a totally different way.
Better yet, the TOTAL WEIGHT added to the bike 37grams, yes... 1.3oz.
That is a 25gram carbon bottle cage, plastic hardware & 3 zipties.... done.
Barry