I culture my cream for buttermaking with store-bought cultured buttermilk (1 cup buttermilk to a gallon of raw milk cream). The butter has very good flavor, I get a much higher yield than with sweet cream, and have found the butter to be very spreadable at room temperature and have yet for it to go rancid, even after a week at room temp. My sweet cream butter was hard to spread and tasted bad after 2-3 days.
I then save the buttermilk from my buttermaking and use it to culture future batches of cream and also use it as a mesophilic culture in cheesemaking. I have used it for Colby, Havarti, cottage cheese, and blue cheese. I haven't tried the blue yet (only six weeks old right now), but all the others have excellent flavor.
I use the same amount as for other mother cultures.