Riha,
For what it is worth, I found that adding a drop or two of rennet to yoghurt gave me results that I liked less than doing without the rennet.
Lately I've been making some really fabulous yoghurt, 1 gallon at a time. I heat a gallon of whole milk to 180º F and hold at that temp for 30 minutes, stirring constantly. (I read a book while stirring!) Then I cool to 120º, pour into a gallon-size glass jar, and set the jar in a yoghurt maker. I ladle some of the milk into a cup with about 1/4 cup of Froyo 2% plain yoghurt (that's the brand that has given me the best results), whisk it together, then pour back into the milk and mix in. I cover the jar with kitchen towels to keep the heat in -- the yoghurt maker was only designed to hold 6-6 oz jars, rather than a whole gallon -- but it seems to work just fine with the gallon jar covered by the towels. 12 hours later, I put it in the fridge. Once it has cooled, I have the most amazing, thick yoghurt -- even unstrained, it is almost as thick as the commercial Greek yoghurt, and the flavor is simply amazing -- smooth and creamy, with just a tiny bit of tartness. Yum! It lasts me about 1 to 1-1/2 weeks.