Ambient humidity actually has little bearing on the thrust of our engines...though it's air density of course that affects them....and that is primarily determined by OAT and elevation. Figure a loss of thrust around 3% for evey 10-degrees F above 59F......and another 3% for every 1000' above sea level. At 4000', then, on a 90-degree day, you might be losing some 21% of thrust relative to standard conditions (STD, that's how the mfgrs' should be listing them). Additionally, of course, your lift is similarly affected....you can see how easily poor model performance gangs up on you at altitude on a hot day! Below sea level, cold weather.......more power.

Split (bifurcated) pipes usually cause a few pounds loss as well.....singles up to a pound. 'Fairest' way to test is to measure the model's slow rolling friction (engine off, level ground) and add that to the max thrust taken during a steady-state slow roll forward....i.e., no fair pushing back...which will inflate figures.

Ray