Design the mechanical parts of an engine , put it together and test it on a test stand measuring the thrust vs RPM on a nice cool temp day, all the time monitoring turbine temps and pressure to ensure that it will run for at least a reasonable overhaul cycle.
Now tweak the control so that it just makes an advertised thrust rating on a hot day and call whatever rpm it takes 100 %
After that your thrust/RPM may vary (your fuel consumption certainly will
)