1: you could use the density altitude with the numbers for ISA, but because your engines are getting hotter air than anticipated by the person who made the table, this is probably a bad idea.
to be on the safe side just use the pressure altitude and ISA+30
2: well if numbers for ISA+30 at 15000ft pressure altitude are in the table then ISA+13 at 15000ft should not be a problem should it?