My two cents, for what it's worth...
I make sheep or cow milk yogurt every week or so, with a culture that I've had going for 3+ years. I started with commercial yoghurt, Straus Family Farms. I heat to 175F, cool to 110-115F, add 1T yogurt/quart of milk. If using sheep's milk, I add 1/5 drop of rennet. (I switch back and forth between milk types, doesn't seem to make a difference.)
Then I set the jars on a heating pad set to low, insulated with towels, and the jars wrapped in towels for +/- 8 hours.
I get a little whey separation and very creamy texture. I just whisk the whey back in when using.