You need to check out the culture descriptions to find one that suits your tartness and viscosity requirements. There are so many different ones.
I have one culture that is tart that I cut (or extended) with dried milk powder as soon as I got it, and put it in the freezer. For that culture, I use 1/8 tsp per 1-2 quarts milk. And, I have another (Y5) that is very mild (almost sweet even) and very thick that comes in little packets. I use 1/8 tsp of that as well (which is 1/2 a packet). I add 1/2 cup dried milk powder to every quart of milk before warming to ensure a thicker, protein-packed yogurt.
You can definitely use your finished yogurt to culture another fresh batch, but each successive batch will be slightly thinner (at least that is what happened to mine). Because I like really thick yogurt, I prefer to inoculate with fresh culture for each make. Especially when you extend the culture, you can make TONS of yogurt for literally pennies.
After 3-4 batches using finished yogurt culture, you could expect to get flavor variations (mutations). But hey, try it 1-2 times and see how it works for you.
Good luck, and let us know how it goes!