Tbm to get over this same problem use a engine that is rated near doulble the hp they require. Then derate that engine in half. Ie 1400 hp derated to 700 hp. This is so when it gets to its cruise alt it has sl profromance ie 700hp if it continues to climb the rated hp now decreases from 700 minus till it achives it selected alt. But as it increases its alt the n1 will increase wf will decrease as will itt. BGT .