Yea I know following a ring on Yew is very hard and lots of people say not worth the effort. If you just follow the contour of the bow and make some effort to stay within a few rings you should be Ok, although I'm sure you know Copper heads will provide no protection.
The way I follow a ring on yew is.... you have to be in bright sunlight and I use my hunting knife as a scrapper, aim for the ring above the one you really want,because you will accidentally go through it. I then sand with my mouse sander down to the next ring. Also the bow is pretty close to final width so there is less surface area to scrape and sand.
I have never made any of my bows longer on the bottom, I tiller 1/8 to 1/4 positive on the bottom and do like you said, make the arrow pass 1.5" above center. Post pictures as you go.
Mark