I havn't played many of the 6max variety, but just in general 60-65% has to be achievable for them to be profitable at any kind of ROI. It's typically depending on the rake a 55-60% ITM required to break even.
They are great for mm though and although can be boring, especially the 10man variety but lots of tables helps with the boredom especially if your raking up profits and building a roll.
What site do you play them on? I have done most of my DON playing on carbon and essentially only for probably the span of a 10 day period back before they killed their lb's.