I had a 420mm fork on my stumpie to, to be honest I have been thinking about that. 420 is supposed to replace an 80mm´s fork which has around 445mm AC and is supposed to have around 10% SAG.
10% sag is a bit on the stiff side for suspension setup, unless you want a nearly rigid ride with with just enough bounce to take the edge off the big hits. I've set up my full-suss with 25% sag front and rear, which is about right to get the full travel on really big hits, without too much bouncing around.
I'm not too worried about getting the geometry exactly right - I mainly plan on using the bike on cycle paths for a bit of evening training, rather than serious off road use.
I was just talking in generally here, I haven't got used to riding with sag acctually, on a dh/am setting it would probarly be ok, but even on my fs I would say I have max 10% of sag. almost none in the rear end but it's an old sid reardamper without some clever platform.
My thoughts were more of the thinking of having 2 forks, one suspendend and one rigid and don't experience to much variation in geometry.
that thing u have created will be a aggressive little thing in the forrest, it's something special riding a lightweight rigid bike with 2.0 tires and a lot of air in the woods. sure it bounces around a bit, but it feels like riding a missile:-)