I have a Ritchey seatpost (don't know the model) and it has a 25 mm setback. I also have a Velo Orange Grand Cru seatpost that has a 30 mm setback. The difference by measurement should be minimal. I picked up a second Brooks saddle, this time in brown, and put it on the Ritchie. I have my first Brooks on the Velo Orange. I noticed a significant difference in the distance between the rear of the seat the the middle of the headset. About 3/4 in. So I switched them. Two questions:

1. Should there really be that much difference between the length of the saddles? I can't believe it's just due to the older one being broken in.

2. Will I notice the difference with the old saddle now being about 3/8 in forward? I realize that's kind of a dumb question because I'll find out as I ride.

Quick questions. Did you purchase the same model of Brooks saddle? If you purchased the same model, did you get the men's or women's version? I know the women's version of the venerable B17 is slightly shorter than the men's model. I believe it's sold as the B17S, but could be off on this. Oh, and I think the B17 narrow is slightly longer than the B17 standard.

IDK should, I'd Measure what IS.

Selle Anatomica has designed their saddle rails to slide back further than Brooks.

I have 2 Fizik Vitesse saddles one a batch shipped through Brompton Sales ,
allows a further setback than the regular ones , because of how the rails are bent,
and that was specified by the OEM customer.

Probably more important than post/rail position is the distance from the saddle to the bar.

Basic Design.. setback is the back half of top tube from Plumb line through the BB.
so same seat tube angle with a bigger frame size will have a larger set back .

Thigh length makes setback need different for individuals knee to hip joints.

Probably more important than the distance from the saddle to the bar is the distance of the saddle to BB.

Difference between seatposts is possible. Some of them have saddle rail clamps that are longer or shorter, which could allow you another few mm. Also, the measurement method (30 mm from where to where?) is not standardized. If you didn't measure them exactly the same way, you could have another few mm error in your description of the two seatposts. How did you measure it?

A difference in saddle height could have had an effect.

I don't want to point to all the things that could have gone wrong, saying you did all these things wrong. I have felt the need in the past to dive into the details, make my measurements better-controlled and try to eliminate procedural errors. A few small mistakes with a few mm each can easily make up a 3/8" (=9.5 mm!) total error, if all your luck was bad. At least, think about whether the seat post measurements are really compatible. And if both companies give you seat post setback measurements, are they really comparable, or are they different?

Another question is, does it really matter? The measurement your body cares about is from the sit-bone support location to the handlebar or perhaps to your customary hand position, with saddle/bar height set right for each saddle. Are the two distances from the rear of the saddle to the widest point the same? If so, what's the distance from the widest point forward to a plumb line down to the BB center? For this one the bike must be leveled. Are the two saddle heights really the same, too?

I would definitely notice the difference a 3/8" change in position makes.

I'm sure Frank with his experience can say a lot more than that about what should be, but I completely agree with measuring and understanding what IS.