How is the recovered heat percentage calculated without an outlet temperature of microturbines flue gas?

HOMER models heat recovery from generators in a very simple way.  It assumes that the generator converts all the fuel energy into electricity and heat, and it asks you to enter the portion of that heat that can be recovered to serve the thermal load.  It does not model the temperature of the heat transfer fluid, nor the efficiency of the heat exchanger.  So it requires you to figure out what proportion of the waste heat can be recovered to serve the load, and to enter that as the 'heat recovery ratio'.