I hope you consider the change the change of TSFC over altitude and Mach number as well as thrust setting. Thrust and TSFC are quite ugly functions of both. For a JT9D I would set the TSFC at common cruise altitudes and speeds to about 0.65 to .7
Thrust decays with altitude, linearly with density. On the other hand it changes with Mach number, strongly depending on the bypass ratio (high bypass > decay; no bypass > increase). For a low (<3.5) bypass engine I would say 20 to 30% is the right ballpark, but exact values are hard to obtain, normally only from manufacturer charts (which are well hidden).