I'm not one of the experts, but here's 2 more of my cents on ERC selfbow design anyway.
ERC is similar to yew in that its heartwood is better in compression and its sapwood is better in tension. So if you have a thin layer of white sapwood over the red heartwood, it is more likely to hold together, shoot well, and not take too much set. If you nicked the back near the bark, it shouldn't matter. I'd pick a ring that'll give you around 1/4" of sapwood over heartwood. The boundary between the two tends to roam across rings, so just pick a ring that's a good compromise, such that sapwood covers the whole back.
I suggest you practice by chasing one or two rings outside of your target ring. ERC requires patience and a delicate touch to follow a ring. A growth ring will tend to have some longitudinal grooves along it, so you'll have some patches of soft whiter wood in these areas. You can either leave it, or scrape it out with a pocketknife.
I've seen some great ERC D-bows (English longbow style), but I tend to make the flatter American longbow style. For these, I use all the width I can, which is usually limited by the amount of clean heartwood I have. It would be nice to have 2" of width or more, but 1 3/4" is fine. I make them around 62-65" long for my 26" draw. If you make it as tall as you are, you'll have plenty of length. I'll either keep the width out about a foot from the fades, or taper it evenly pyramid-style to the tips. ERC is very soft, so you'll probably want to keep the tips about 1/2" wide, or else add overlays. Tiller slowly and gently!
I'm finishing one up now. If it doesn't turn out too ugly, I'll post it soon.
-Tom