Hi Mark,
Arion's mast is also raked forward by three degrees. It was not uncommon on Chinese junks, and many yacht schooners rake their foremasts also. I did it because it has the advantage of holding the sail out when the sheet is well eased on a broad reach or run, especially in light weather, and because I wanted to move the CE a little further forward from the position it would have been with a vertical rake. My mast partners could not go any further forward. In Practical Junk Rig (PJR, a good investment for a junk sailor) Hasler and McLeod recommend a vertical mast. Either is good. Raking the mast aft is not, as the weight of yard and battens will make the sail swing inboard in light conditions. In a swell, they do crash around sometimes, but in smooth water, a vertical or forward rake will hold the sail out and the boat will ghost downwind in the lightest breath of air.
Positioning the mast is easy. First draw your sail plan, then superimpose it over a drawing of the bermudian sail plan so that the centre of effort of both is the same, or the junk's CE is slightly further forward perhaps. The mast position should be about 10 to 15% abaft the luff of the junk sail. I'd recommend reading PJR before making any firm decisions, to build up some theoretical knowledge.
Your final decision will be to decide what kind of junk sail you want (there are a few options, though I favour the "Arne modified" Hasler McLeod sail) and whether it is to be flat-cut, cambered, hinge-battened etc.