I used to do swings as part of my job as an avionics chappie. They really aren't that hard to do, it's all available on the net these days, all you need to do is borrow a landing compass and tripod.
It would seem a bit odd that it's always out by the same amount though, they are usually more 'out' on certain headings than others. Having said that, a dash mounted E2 will go out of true depending on how far forward or back you have the stick...