Set the throttle trim to zero - always. The idea is that you don't want any motor RPM when the stick is at zero. Theoretically, all trims should be centred for all control surfaces (Pitch, Aile, Elev and Rudd) and zero for Throttle.

You need to start with setting your zero pitch at mid-stick. To do this, make sure you either use throttle HOLD or the preferred option is to disconnect a couple of motor leads and then make sure the F. Mode switch is in Pos 1. raising the stick to the mid point 50% in and 50% out and set zero.

Too much to go into here but there are plenty of resources available to you on the net. Search for the DX6i on YouTube and if it is for a Blade 400 search for the Blade 400 bible by Slyster.