What formula to calculate copper foil thickness and width on transformer construction for secondary, because PIXls design HiperTFS-2 accept anything in this field.

I have this one:

Area[mils^2] = (Current[Amps]/(k*(Temp_Rise[deg. C])^b))^(1/c)

Then, the Width is calculated:

Width[mils] = Area[mils^2]/(Thickness[oz]*1.378[mils/oz])

I dont know, because ie: PI expert for TopSwitch[JX] show in example for 5A, 11.20mm width of 5mil(0.13mm)Thickness ,
in this formula above for 5A result is only 1.94mm.

Why i ask the real formula for that.