The Tarn has a 72 seat angle too, so that will add extra length to the ett, which is why its such a crappy figure for sizing given you’ll put the saddle in the same place by sliding it on the rails.
Im 6ft4 and only XL have ever given my much reach.
My previous trail bike had a reach of ~430mm, ran it with a 110mm stem usually.
Current one is ~450mm reach, run with 80mm stem.
My fat bike is an XXL with 467mm reach, I run it with a 70mm stem.
If I was to get a tarn (or high latitude, sell the frame separate Genesis!!!) id probably just get the Large as it is on the long side, but then this is the first time im not just automatically looking at an XL 🙂