What you need to do is have the servo arm at different than a 90 degree angle to the pushrod. This way when the servo turns it will have more travel in one direction than the other. Actually the servo moves the same amount but because of the angle to the pushrod it creates a situation where there is more resulting movement one way than the other.

I don't think I explained this very well but it is a common situation in helis where all control rods should be at 90 degrees to the servo arm so as to eliminate differential.