Fusing question

I am building a system for a friend that is going to have an 800 watt rms (@14.4v) monoblock fro ist subs and a 4 x 60 watt rms (@14.4v) amp for the speakers.  some sources say that the best way to determine the fuse value is to take the 12v rms wattage and divide it by 6. unfortunately I don't have the 12v specs handy.  Can I just divide the 240 and 800 watts by 7.2 and use the resulting 33.33 and  111.11 amps as the current values?  Also, to fuse  the amps on individual fuses I am going to need to use ANL fuses as AGU only goes up to 80 amps (wanting to use a fused distribution block, don't see any combo blocks out there).  Unfortunately I can only seem to find 100 amp and 150 amp fuses.  I need to use a 40 amp fuse for the 4 channel, but ANL only goes as low as 60 amps.  Any suggestions as to a course of action? (By the way I'm running a single 1/0 wire back from the battery to the distribution block and 4 and 8 guage wires to the amps)

How is that possible that the current draw is that low?  The 800 watt amplifier has 120 amps worth of fuses on it (four 30 ampers to be exact).  Won't the fuse blow when the current draw spikes over the 800 watts on really hard hitting bass notes?  Also, what do you think of Maxi fuse systems?