Pot odds are calculated by the money in the pot, being put into the pot and what you are going to be putting in. In your example the Pot is $50, he puts in $30. So now that makes $80.00, divide that by what you have to put in which is another $30.00 and that is roughly 2.666:1.
For flush draws (which are more likely) I like somewhere around 6:1 pot odds. I may to 5:1 once in a while, but usually has to be 6:1 or so. For straight draws (which are less likely) I like at least 8:1. This iss becuase the odds of hitting the striaght are low, and therefore you need larger pot odds.