Our side channel spec sheet is
here.
Due to the abnormalities of window installation, you will want to order the side channel slightly larger than needed and trim it to get the best fit. The length of the side channel will be determined by the distance from the window sill to the pocket or fascia that it meets at the top of the window (see spec sheet).
There is no deduction in SCT if you have side channel. This is assuming a 2.5" side channel aligned with the edge of the bracket/pocket. This would insert 1.75" of fabric into the channel, ensuring a good light seal.
The seal on the window sill could be accomplished with the hembar with wool pile, our sill angle, or something you fashion on your own.