Can't see how it would be important to match the radius in any exact way, though it would tend to happen naturally, since that groove is already started for you. The main point is to get steel out of the way so you have less steel surface to sharpen on.
You want to keep everything nice and straight. If the honing strip, or the area behind the bevel, are not darned close to even and straight along the length of the blade, then honing will have less effect in the areas of more steel, and less effect in the areas of less. My guess would be that this is already happening to some extent, the way the razor is now, honing having less effect toward the toe. Yes, you can compensate for this, if you're paying close attention, but why not get it straight and make things easy?
Sure, try the half-round file. I've used them, but they wandered too much on me, and I couldn't keep everything straight, and I kept putting scratches in the parts I didn't want scratches in.. Maybe that's just me. The slipstone sounds better, easier to control, but the ones I've seen taper, which complicates things a bit. Still, I'm tempted to try it, if only to have something that would work for me besides sandpaper wrapped around a pencil or dowel. Or maybe there are non-tapering slipstones.
