RE: Magnum CG - ?????
You are right... we are flying with "forward" CG's, at least we are when you compare things to straight wing designs. I don't quite know why, but this seems to be the case for most delta and high sweep wing designs.
If you take a look at a Diamond Dust for instance, and you determine the MAC the same way, you end up thinking that the plane will be nose heavy... yet it flies just fine with the (most rearward) CG as indicated on the plans.
Must be something about delta wings...?
But rest assured, if you balance your Magnum at 135-140 mm, it will fly good.