I am coming from a nagauta perspective, but for this it won’t be much different, I think. Just hold the point of the bachi against the string and look at the angle. If the upstroke presents the flat side of the bachi to the string, it will catch. Lower the tail of the bachi a bit so that only the thin edge of the bachi point hits the string. I agree that a relaxed feel is necessary. And, it is better to under-play the upstroke–I mean getting too far away for the upstroke to sound, than getting in too close, which will cause you to hang up. For a sustained alternation, start too far out and slowly move in. It really doesn’t take much to get it to sound. Once you get used to this, you will be able to find the sweet spot right away. Also, make sure your position for holding the bachi and for your arm on the body of the shami are stable and the same every time. Finally, this might be just a nagauta technique, but some players put the little finger of the bachi hand against the skin–this gives you some reference to always be in the same spot (nagauta only hits one spot, so the ‘moving target’ technique of Tsugaru might make this impossible–I dunno).