Do you make yoghurt? How?
Stir together a quart of milk with 1/3 cup milk powder, if desired. If using frozen yogurt starter, set yogurt cubes out to thaw.
Heat milk to 180 degrees, stirring constantly. Use a candy thermometer to measure temperature.
Remove milk from heat; let cool to 115 degrees. This takes about 1 to 2 hours, depending on the temperature of the rest of the house.
Stir in 2 tbsp of yogurt starter. Mix thoroughly.
Pour the milk/yogurt mixture into sterilized jars. (Whenever I do it, it's a bit more than a quart, so have an extra jar ready.) Incubate for 4-12 hours, or until it is set (i.e. looks like yogurt, not milk). The longer you incubate, the more sour the yogurt will be. Refrigerate until cool.
This is more or less the exact recipe that I've been using to make laban (the Lebanese name for yogurt) for years. Kudos on advising that increased incubation time makes the yogurt more sour. The only thing I'd add is a simple few steps that would allow you to make labna (yogurt cheese):
Recipe: Labna
1. Make a bag using three to four layers of cheese cloth
2. Place two to three cups of yogurt to the center of the cloth/bag
3. Tie up the bag to one of the shelves in your fridge with a bowl under it to catch the whey.
4. Let it hang for a day or two, remove labna from cheese cloth.
5. Form into balls, salt lightly, and store.
If you're using commercial yogurt to make homemade yogurt, it's not really homemade yogurt. Use a starter. It makes a difference.