I would like to say one stay away from metal servo arms unless they are designed to "break away".
Second double check your linkages. They may not be what they should be.
Third what's supposed to happen as smoggie said, first you 90 degree the arms mechanically first, then in the FBL unit trim it to 90 if needed. Next, your swash should be close to mid travel on the main shaft.
Fourth, check the link geometry on the swash, if it's not what you want you may need to adjust the servo to swash links.
Fifth, check blade pitch at miss stick. At this point you need to make sure your as close to zero degrees as possible. You maybe out by a tenth or so. Do not worry about the few thenths of a degree.
If all of that looks good then go and set 6°. As Tony said not everyone is going to get 6°.