About how much room between tire and saddle rails will I need to fit the Carradice Nelson with the standard expedition bagman?

Sorry if this is answered elsewhere- I swear I saw the answer to this somewhere, but now I can't find it.

Need more information. What size is your frame, seat tube center-center? What is your saddle height, BB to seat top? On my bike, it's about 11" vertically from the saddle rails to the rear tire, but my frame is a size 57 cm with 75 cm saddle height, and I'm using Carradice's smallest bag (Barley). The Bagman should keep the seatbag from hitting your tire on a mid to large size frame, but I'm not sure about a small frame.

I ride a 21" frame with maybe 6" of seatpost showing, and my Carradace Nelson sits comfortably on top of my rear rack when using the SQR Uplift - but I think that carries the bag a little higher than a Bagman. So really, that doesn't tell you anything. If you are worried about it, and have a small frame, you could get a Lowsaddle Longflap bag (about the same overall capacity as a Nelson), and use the regular Bagman instead of the expedition.

I thought there was a fixed measurement between the rails and tire that was needed to fit the expedition bagman. From the wallbike website it looks like you need at least 11", but that's for a nelson without a bagman.
Anyway, I just went ahead and ordered the lowsaddle, because wiggle sold out of the regular nelson while I was debating.

I think you made a good choice . Are you getting a expedition bagman? I have a nelson/exped. Combo & like it a lot, very secure . But it does need 10"-11" from the Brooks bag loops to the tire for clearance. I would like to hear how a standard bagman would work with a Nelson bag . That may give more clearance if the bag still fits well.

Cheers