i was looking at the optimum &maximum flight level charts for the airbus a330 and couldn't figure out "max and opt alt (isa+20) ?Example. Crz speed 0.80, weight 190,000 kilos or slighlty less than 420,00lbs max and opt alt (isa+20) would be 37,000.How do you calculate that isa+20 ?thanks

That's ISA temperature for that altitude plus 20 degrees.