My potting mix ratio is a bag of soil conditioner (2 cu ft of pine fines), a smaller bag of compost (cow or mushroom) and a smaller bag of potting soil. I get the cheaper brands at Lowes. Will sometimes add more potting soil to a pot if I want it to hold more water (for summer annuals). This mix drains pretty quickly and allows good root growth. Easy to pull weeds that try to find a home. Most of my black pots are 3 gallon, but will use smaller for the dwarfs and some arilbreds.
The manure helps hold moisture in my mix and adds organic matter. It doesn't hold as much water as potting soil. I have substituted top soil for manure and/or potting soil. It contains clay and stick pieces that added little moisture retention. I had to water more so switched back to my usual mix.
teresa, i grow many of my irises in pots, and use a commercial cactus mix along with potting soil. it's not an exact science, but my mix is about 1/2 @ 1/2. i sprinkle in osmocote and initially water with a very weak solution of miracle grow.
