[zennmotion]

There's probably different (and maybe easier?) ways to replicate saddle setback, but I use a construction bubble level (just a cheap 24 inch plastic thing from Home Depot, is this why my DIY remodel projects are off-square?) and a plumb line (piece of string with a fishing sinker or just a large nut tied to the end of a 4ft long piece of string) and a tape measure. You might need an extra pair of hands-

1) dangle the plumb bob so the string exactly lines up with the BB center
2) rest one end of the bubble level on the tip of the saddle to be sure you're measuring a horizontal line
3)With the tape measure, measure the distance between the saddle tip and the string along the horizontal bubble level

This works if you're using the same saddle on both bikes. If your saddles are different then the next best thing is to measure along a virtual horizontal line to the center of the seatpost to compare setbacks between two frames, but then there is some minor, though imprecise, adjustment to set a different saddle in the "same" position- no two saddle models feel the same. If your top tube is horizontal, no need for the bubble level, just mark where the string touches the top tube for the center point

Like I said, there's probably an easier way, but this uses what I have handy near my work bench.