I crunched a bunch of numbers on this last year, and came up with a very similar number to yours - 300 head.  I made the assumption that harvested feed would be purchased or custom harvested (if you're doing all your own you should be charging yourself the opportunity cost anyhow) so that all family labor could focus on the cows.  At 20% equity in the cows and land take home income would be $60,000.  With cows and ground paid off take home income would be $300,000.  I'm pretty sure I based prices on the last 5 years average of feed and cattle markets.