Well, I don't know that I NEED to go 4" deep. I know the point weight that I desire to use is 150-190 grains, and these first attempts with the long bit were 1)to see if it wanders out the side of the shaft, and 2)to see how it flies.
My son's made up some with the short bit that came with the jig, free-handing the holes when the guide bushing bottomed him out. Was less than 3" deep. They shot ok for him, and so he shot a few of them into a hard stump....just to see if it strengthened the shaft behind the point. 58# bow, less than 20 yd - 4 or 5 shots, a couple bent very slightly right behind the point, and he was able to straighten them and then shot them into it again. Not that they were intact internally, but none broke. That was an eye-opener for me. This won't make cedar almost indestructible like the hardwoods, but it does seem to improve it.
If I knew a machinist who'd work for nothing(!), I'd have a more accurate jig made. A longer and smaller diameter guide bushing hole would tighten the concentricity of the hole and how closely it approximates the centerline of the shaft. Who knows how much those things affect flight or accuracy?
The only test I can think of is to find out whether it can be tuned to bare shaft well, and what spine that requires. Has anyone found that it significantly affects tuning -that is, did it change the spine by very much?
Appreciate everyone's thoughts so far.