You don't need any ackerman angle unless the cables are crossed and the bellcrank attachement points are in front of the pivot, I will dig out the instruction manual from the QQ Yak which shows this in deatil and post as soon as I find it.

Mike

EDIT: found it, I think its all self explanatory, the 7deg angle has to be equall at both ends. And by the way its not inportant to have slack in the non pulling cable, its just that most can't do it any other way so they satisfy there own ego by saying its neccesary, now watch the fur fly. If your cables are not crossed and the servo arm is the same width as the rudder horn then it should all stay alligned and the cable tension should not increase or deacrease during operation.