Using external viewfinders are always just estimates. But the wider it gets, the less parralex.
I used to use 28mm and 35mm external finders on a leica iiig. The 28mm is tolerable, but the 35mm missed by noticable margins. However being on film sometimes you just forget your original intended framing.
The zeiss 25/28 is the best finder i have seen, and i have used many 28mm finder from voigtlander to grd finders, leica turrets and canons.
Alternatively, you can ask leica sg to upgrade your finder to a .58x one, which is the best for 28mm. Upgrade to mp finder was 700... Not sure about .58 though.