Most PSU Guru's say add the rails together and subtract 30%.ok so for a four rail psu you only add 3 rails together to get accurate picture of amp supply
but what do you do if its a two rail psu? is that still 75% of the total?
For example 18 amps + 18 amps = 36 amps .
36 X.30 =10.8 amps; 36 amps - 10.8 amps leaves you with 25.2 amps.