Engine RPM 2000, engage red lever slowly until engine rpms drop down to 1300 to 1350 (engine sounds like it wants to quit) and stop engagement, let rotors catch up until around 85-95rpms and then you should be able to engage all the way down. Now you can slowly add throttle, at 100 RPM you can pull stick back to neutral and at 130rpm go full aft. around 2700 you should be seeing close/around 175rpm rotor rpms (do not exceed 3100 engine rpms). 175 rpm what I launch at as I am using a hard surface runway that is long and it is plenty.
Following this technique will prevent the belt from slipping and heating up and melting the 3D printed part.
PS - you should or will feel the stick pull right some, make sure you center stick before launching or you will drift right.
This is what I am doing based on some discussion with Fusioncopter, please share your technique/experiences so we can all learn.