Alex,
I try to get some ricotta regardless of the cheese type and the procedure is similar to yours. I am using this cloth called calico to gather those semolina sized curds and from 8 litres I've got at most 800g once.
Add 1 litre of milk (usually UHT),
Heat it up to 95C,
Add the 1/4cup vinegar (for 8 litres of content ),
Turn off the heat, let it cool
Go and watch a movie,
Come back and strain it through Calico
Calico is almost like sheet or pillow cover. It drains slowly and takes time but you don't loose the precious little curds. Once I kept the whey about a day on the bench and I think the acidity has risen too much that ricotta is appeared on top without heating though must be different protein because Albumin protein does not come out without heating (correct me guys, if I am wrong)
Also
http://en.wikipedia.org/wiki/Ricotta says, it needs to be acidified first.