Further use has led me to think 40% risks getting a process called 'system_server' stuck at using ~100% of a core. I'm not sure if it recovers eventually, but if it does it seems to take a long time to do so. On the plus side, you have to be running a fair few things to get it there.

Guess I'll try 30%...

[edit - although note that system_server seems to be part of Alien Dalvik, so if you don't use that, *maybe* 40% will work)