I get 59 kW. I think you misinterpreted the rule for above 26 ranges (i.e., 15 kW+ 1 kw for each range). I interpret that to mean you start with 15 kW, then add 36 kW because you have 36 ranges. That gives you 51 kW. Then you add 15% because each range is 3 kW over the 12 kW value, so you need to add 3 time 5 %. That brings another 7.65 kW to the party. The 0.65 is more than half, so I round up. Finally, 51 kW plus 8 kW gives the total of 59 kW.
But ignoring the math for a moment, don't you think it unreasonable that a total connected load of 540 kW (15 times 36) could be counted, after applying demand factors, as only 18 kW? If I had come up with that result, I would think I must have done something wrong.
Charles E. Beck, P.E., Seattle
Comments based on 2008 NEC unless otherwise noted.