You got just about as much butchered meat back from your 4 Zwartbles lambs as I did from my 4 Castlemilk Moorits 2-year-old wethers. The Castlemilks were very fit, but even so I would have expected you to get more from your Zwartbles - the Zwartbles is a much larger sheep, and should have more muscle (better conformation as far as the meat industry is concerned) than the Castlemilks.
If you get a chance next year, go to one of the big sheep shows (North Sheep, South Sheep, etc) where Eblex have a stand and get yourself a demo and hands-on go at assessing lambs' condition.
Generally liveweight to deadweight is around 40-50% - a well-caked Texel at the higher end of that, a grass-reared hill lamb at the lower. There'll be more losses in the butchering, especially if you have any of the bones taken out, of course. And more losses if the lamb is fat, as the butcher will trim as much fat as possible.