There is no reason to make the linkage with the servo arm angled at the neutral point unless you need the full top-to-bottom range of the slider. You should be able to arrange the arms and linkages so that isn't necessary. You can build some differential in by angling the arm but it isn't really worth it with flaps as they deploy in the same direction. Set it up perpendicular. I usually make the slider neutral at the mid-point, also. It's a personal preferance (move them up and they're spoilers and that's just as handy at times).