After Lars video about rear foot width, I dropped an email to Korua to know this spec, for TF50 and 54
They replied me that the width at the back binding inserts are 265mm for TF50 and 275mm TF54.
When I was waiting the reply (I was not sure they were going to reply me at such a nerd question) I took the specs of both the TF50 and TF54:
TF50
Waist width: 252
Tail Width: 278
I did 278 - 252 = 26 mm of difference from the smallest width and the tail width
So, supposing that more or less the back foot insert packs are roughly at the midpoint between the 252 waist width and the widest point of the tail (yes there is a setback, but few millimiters of difference in width are negligible), I did this:
26mm/2 = 13 mm
252 + 13 mm = 265mm
for the TF54 I got 274mm
that was a rough estimation, but I was spot on.
So about backfoot width, with the TF50 I'm in the same ballpark of my other boards, that are spot on as Lars suggested