well lets do a lesson here take the fuse rating say my pioneer amp
30+30= 60 , 60x 14.4 = 834 x .5 (because im an a/b class) = 432watts maxx rms at 4 ohms
so u take the amp u want multiply the total fuse rating by 14.4 and the multiply by either .7 or .5, .7 is for d class and .5 is for a/b