Patriotic, the maths doesnt stack up like that.
The total power requirement is subject to load factoring for inductive loads (start up inrush currents) and other variables.
Otherwise,gtx960 sli would be the normal requirement plus 120 watts would it not? (520?)
It isnt of course, and that is why A.M.D. recommend a 500 watt supply, some sites recommend a 550 watt supply, and I would recommend nothing less than a tier 2 550 watt supply for an r9-380.
Short story is, no matter how good your arithmetic, the people at the gpu manufacturer know a smidge more about it than you do so follow their recommendations.