how far out the servo horn should the ball link be? 1st, 2nd or 3rd hole out? using the star horn with four sides that's included with the 9650 oh and also, the tail drifts VERY slightly to the left, is this normal or should it be dead center and shouldn't move at all? (being in heading hold mode). Thanks!

Its for a trex 450SE V2

I also have a 9650 401 combo..use the horn with the least amount of distance...In other words get the ball link closest to the servo shaft as possible..I used the round horn and put the ball in the closest hole to the servo shaft...As far as the Tail drifting, you shouldnt have any...Check out all of Finless bobs vids...Are you sure your gyro is in HH mode...Make sure your light on your gyro is always always on and not flashing...

it will drift if the rudder isnt mechanically neutral. i have been running this combo since i've started and it holds rock solid. like dirt dawg said "check out finless bob videos at helifreak. he explains it all with visuals.

I already did. everything is mechanically centered. tomorrow I'll change the ball link position and see what happens

The ball link position is easy to figure out. Plug your rudder servo directly into your receiver so your gyro will not effect the throw.
Move the rudder stick in both directions, with the link disconnected, and determine, by holding the link up to the arm, which hole will give you the correct amount of movement to get full throw in each direction but not bind. (if you get real close but just a hair too much throw you can use end point adjustments in your radio to fine tune)

You don't use end point adjustment to set rudder servo throw, you use the gyro limit pot.

For the trex put the gyro in rate mode, center the tail pitch slider, rotate the arm 90 degrees at a time till you find the one thats straight up and down. Use the closest hole to the servo center.

Adjust the servo position and or linkage length to center the tail pitch slider the use the limit pot to get the most throw you can without binding.

Set it to heading hold and go fly. Use the rudder end points to adjust the piro rates, you can go over 100 and you may have them slightly different to get symetrical piro rates.

One recommendation: Use the round servo arm.....the majority of people I see use the round one including myself with no issues.

Skarn

ok so I installed the round servo horn putting the ball link on the outer hole. I took the heli for a spind and it was actually worst[X(]. patteries are charging up but I will increase the gain or sens to see what happens. Read the 401 FAQ and there's a question addressing that particular problem put after a few flight and it sais to actually put two pieces of foam instead of one to reduce vibration and that might do the trick, if more gain doesn't do it I'll try that, any other tips?

Use the inner hole and a higher limit. I doubt you have a vibration problem on an electric heli unless its shaking like a scared dog.