I think DONs are great for building up your BR. I play Fifty50 on Pokerstars which is basically double or nothing. Only difference is that you need to try accumulate chips, because you get extra money for them. If you're good you can pay 1.50 buy-in and walk away with around 4.70$. As for strategy I try to take as little risks as possible and at first I'm playing tight. During bubble period I try to steal when given opportunity..... and basically just survive. If you play regular DONs, there's no reason to rush and all you should aim to do is survive as there's no extra money for player with extra chips. I'm not sure about ideal winning rate, but I managed to win let's say 3 out of 5. Variance in these is not that high, so it's all good if you're solid player, you can build up on these.