I used the 21mm Zeiss finder with the 24 Lux in Germany. The framing was actually relatively accurate. I have a 25/28 finder, but didn't bring it with me. Based on what I saw with the 21 finder, I'd venture to guess that the 25 frame lines might give you a fair amount more than what you are seeing through the finder. The 21 finder was giving me just a squeak less than what I was seeing, but not much.
Another finder to consider would be the D-Lux 4 viewfinder. Actually pretty nice and it is 24mm. Not as solid as the new brass 21 and 24 Leica finders, but still optically good and more compact/lower profile than the Zeiss.
I also took our a 21/24/28 Leica vario finder to play with today. There are no brightlines but the zoom feature is nice and the image is undistorted and bright.
Options, options, options. Good luck.
David