If the two bikes (the one that fits, and the one you are setting up) are going to use the same or very similar saddles, you can use measurement. On the one that fits, use a plumb line and a ruiler to see the distance between the nose of your saddle and the BB. See if you can position the saddle at that position on the new bike, with correct saddle height. This will tell you if you really need setback or not on the new bike.

The two bikes could have different seat tube angles, but this technique sidesteps that issue and directly checks the butt/pedal/BB relationship. The key to finding the best startign point is to get your contact points into the same positions on both bikes. I've found this strategy to work with my 4 bikes.

Road Fan