I suspect the Boeing "hand on brakes" method is a little vague, I would think that 50'C would feel hot to a naked hand, yet the wheels and brakes could easily get that hot in the sunshine in our hot summers (on the ramp at MEL or SYD or any city in oz).
An interesting note, I once saw a low-time-on-type F/O heat the brakes up very hot during the long taxy at BNE in an Airbus (can't recall model) the screen was showing high 400s and low 500s for all the wheels on the MLG. The Capt., kept telling him to allow the plane to build up speed and then slow the plane down with short applications of the brakes, but the F/O must have been nervous about his taxy speed because he kept those brakes dragging most of the way down that long taxy way.
I'm glad we didn't have an RTO as I suspect we might have had a little brake fade.
Cheers,
ABX