I believe the main idea is:
there was actually cold stuff (hopefully fuel) in these main tanks, and obviously enough for another 2 miles.
But still, I’ll be interested if Green-dot is able to produce an estimate.
I would not bet on any figure, but to me, the area looks pretty tiny, at least more tiny that the one from ANA when blue sky (and Metar confirms temp / dew point) was not optimal for that frost formation. To have access to that ANA remaining fuel could provide some direction of thinking …
Regarding BA038, humidity was effectively more important with that broken at 1000 ft confirmed by a 2 deg dew point spread on surface.
I do agree that TAT has an influence, but not that much in this case when you consider this remark:
“clear outline of frost starting at the rib, separating wing tank(s) from center wing tank”
Nevertheless I do consider the limited cold fuel influence on the outboard as volume is lesser, but I still find the outline quite marked.
Just one more thought:
Considering the huge size of the CWT close to 100000 liters (and also the architecture, as I understand it the tank is not in direct contact with the fairing skin ?) I’m not sure even 10000 liters would be made visible by any frost on the left or right side of that tank, as that volume would be exclusively in the lower part, just in the center of that center tank … ?