I decided to invest in a reducing back for my 8x10, and I have finally had time to play with it.
Out of the box, and with a flat lens board, I think the minimum focal length is 110-120mm. The 5x4 film plane is some 10-15mm further back than the 8x10 film plane, and the minimum extension is set by the closest front standard mounting. Being a curious type, I fabricated a small bracket to let me mount the front standard some 33mm further back, and that allowed my 90mm f5.6 Caltar HR to focus at infinity when on it's flat Linhof/Wista board.
Using a recessed Sinar board ought to be enough, but I'm not about to remount my 90mm every time I want to use it on a different camera. The bellows are the limiting factor with a short lens like a 90mm. Swing and tilt are reasonable, but rise/fall is very limited. A recessed board would help here, too.
The rationale for using a reducing back on an 8x10 is a) film stock, b) effective image size on the film (short and long are easier to obtain on the smaller format), and c) simplifying equipment on an outing.
For me, a) does not really apply. I tend to use Delta 100 over HP5 in 5x4, and just HP5 in 8x10, but there is nothing I want to use that I can't get. Case (b) has some merit - I only have one lens for the 8x10 format, but longer and wider options at 5x4. I can enlarge 8x10, but it is more of an exercise than 5x4. And I am not interested in a digital second stage. Case (c) may actually be my best use, since it is a small extra effort to include some 5x4 holders and an extra
lens with the reducing back would allow for a change in plans. If I set
out with the 5x4, it is unlikely I'd hanker for the 8x10. Do I need it? Probably not. But then it is arguable that I don't /need/ the 8x10!
The picture shows my 90mm with my quick adapter to get infinity focus - a couple of pieces of scrap oak glued together, a hold of the normal screw knob, and a coach bolt for the new swing/shift pivot. You can see how compressed the bellows are. The lens panel is a modified Sinar-Wista adapter.