Metatranscriptomics Extraction
Consider doing triplicates of just 4 combinations of cycle/dilution: low+low, low+high, high+ low, high+high.  Simpler.  
Most important: prove you can extract DNA from a clean room.  If you extract from a real clean room and get no result from the PCR, maybe it is because you failed to collect anything or maybe nothing was there.   Idea - spray a known quantity of DNA into a room that will serve as your sham clean room.  Then do your extraction protocol.  You can test the cycle / dilution parameters with the sham clean room extraction and have those variables optimized for when you collect from a real "wild" clean room you did not dope with DNA.