Originally Posted by
Iride01
Setback probably should be measured from a vertical off the BB. If your bikes have different seat tube angles then you won't get the saddle in the same relation to the BB by measuring from the seat post.
You could also just set your reach to your hand position on the bars, but if everything else isn't the same, then still your saddle distance behind the BB might make things feel drastically different for you.
I measure setback by placing my bike in the corner of a room, measuring from the wall where the rear tire is touching to the BB, and then to the saddle (same saddle each bike), I'm less concerned with the actual setback # for a bike, then the difference between them (with one bike feeling spot-on).