I'm far from being an expert at crown but I did figure it out and my job came out very nice. So...
You're talking 90 degree INSIDE CORNERS, right? As far as I understand, the spring angle wouldn't have any bearing on it ("spring angle" being the angle of the mould's face if installed, right?). I never even bothered with it doing butt/cope. Even if your "90 degree" inside corners are off a tad, butt/cope covers that up. I start with a piece of moulding that is a little longer than the span to cover. I just cut my miters (always upside down on the saw) at 45 degrees (by swinging the saw blade, not tilting it). Make sure your mould is firmly sitting at 90 degrees on the table and fence (again, upside down). This is an INSIDE MITER cut for INSIDE CORNERS. Once cut, you then take a coping saw, an electric sander and/or a half-round file and steeply back-cut or clearance the area from the edge of the curves of the crown's design back into the mat'l behind it. This reveals the profile of the crown's design and makes a knife edge that will butt up to the adjacent piece. The latter piece is cut 90 degrees and butted up to one side of the inside corner first. The miter cut piece I described is then attached to it. Once I've made the mitered end, I then mark how long I need to cut it off and only do so then--gradually cut to get a snug fit. Crown is confusing at first and I hope this helps and didn't confuse you. Here's a couple links with some pics that really helped me.
(nice aerial drwg. of butts and miters)
(good pics of the profile back cutting)
http://www.extremehowto.com/xh/artic...ticle_id=60354 (small but good pics of the saw settings and look of the diff miters)