Calculation of max possible current load from voltage multiplier connected to PSU 12V rail
Voltage and current can be multiplied, but power plows through.  You have 400W available.  Let's assume the DC-DC converter is 75% efficient; that means you have 300W available to the output.  At 60V, 300W is 5A.  Plug in the real efficiency and you'll get the right answer.  (Note that efficiency may vary depending on the amount of current you're drawing, and the value you're interested in is close to the 5A level.)