If it can shrink that much, does that mean I have plan for a radically different size of material if I wash it before making something.
If I can't actually get my hands on the wool before I buy, I'll buy just a metre and I'll make up something like hooded capelet with it, but, before I sew anything from it, I'll try washing some of it first.
Virtually all wool will be fine with a cold overnight soak, then a gentle wash through with shampoo (it's meant for hair, it meant not to destroy hair, wool is just funny hair, it'll do fine, and cheap 99p a bottle supermarket stuff is fine for washing clothing) and lots of rinsing. Do not thermal shock it, by going from hot water to cold or cold to hot. Let it drain, lay it on the garden bench, or on the bottom of the bath or shower, and when as much water as possible has drained out, drape it over the biggest towel you can find and roll it up, wring it and that'll take out enough water to let you lay it out and dry flat. It ought not shrink.
If you wash it hot and soapy with lots of moving around, if it's going to shrink, that'll do it. Machine washed on something just like 40 ˚ should do it for you too, especially if it spins afterwards.
On the whole bushshirts rarely really need washing anyway, maybe if you can't thole the smell of woodsmoke permeating other coats and jackets in the press though, or if you get it bloody. Mud and the like generally brush out fine.
So, the answer to your question is that it depends on the wool and the use you'll give the garment you intend to make.
Sorry, but that's the best reply I can make on this.
M