The SCT will always be the most up to date place to find current size and fabric information for Lutron shades. For Triathlon roller shades with a fascia, the light gaps will be 7/16" on each side. Triathlon rollers without fascia, or ordered as inside mount, will have different light gaps.
The current Triathlon and Serena Spec Sheet can be found at
www.lutron.com (type 085312 in the search to go directly to it). On page two it shows both back cover and fascia dimensions. The width is simply called "Shade Width". This will be the overall width of the shade (or "B to B" in the SCT). Unfortunately, the light gaps are not mentioned in this document.