On most Rotax 912/914 installs in Kitfox airframes, there isn’t a single “factory set” exhaust length from engine flange to muffler inlet. It’s typically a custom fit within the Kitfox exhaust system geometry. What matters most is keeping smooth bends, proper slip joints, and adequate support so there’s no stress on the heads or cracking at the flanges. If you can share which muffler system you’re running, someone may be able to compare it to a known Kitfox F23 installation.