What I did was to use the Voltage Drop Calculator here http://www.electrician2.com/calculat...cpd_ver_1.html
with Cu, 50 feet, 120 1ph, #6, 26244 for cir mils for #6, and k=12.9 for Cu
Solved it iteratively for a 3% drop and got 74 A. I divided that by (1/0.8) and got 59.2.Then upsized to 60A
In esscence 74 Amps in #6 copper at 50 feet away continuous will cause a 3% drop.
Apply the derating and up-size, so, that the wire and the protection element are correct and get 60 amps for the overload protection.
A 60 amp breaker cannot handle 60 Amps continuous by design and you have to derate the breaker to get the maximum continuous load of 48 Amps.
Both ways to me make sense. I never think like someone else does. The iterative calculator tool made it easy. Oh well!
For reference, the FUNDAMENTAL formulas are R=pL/A or Resistance = resitivity * length divided by cross-secional area and ohms law with modifications that make the numbers convient.