The math is absolutely correct on this, however real world usage may be considerably lower. I run 2x 328s, 2x 18sai, 2x ult15's, 1x 32r, dbx pa2 and 2xafs2, a variety of lights and whatever a band will bring and rarely go above 13-14 amps. I use a 20 amp furman pl-pro with a volt/amp meter on the back to keep an eye on things during events.
The 18.2 amps in the math example is assuming you're running full bore all the time.