How do you go about running the mill at exactly ISA? Totally impractical in the real world!
For performance acceptance testing, airlines have test cells from SL (AMS or FCO for example) to over 5000 ft (Joburg), and Montreal gets real cold much of the year, and Abu Dhabi real hot. So a method is required to correct the data from whatever non-standard conditions exist to ISA SL. Clarence has posted such a chart above. (Actually, in the test cell a humidity correction is applied too, a refinement which is omitted for the on-wing case)
So - run the engine per the chart, with appropriate temp & alt. corrections, and it will produce rated thrust.