I use one third compost, one third something organic-y like peat moss or a soil conditioner made of ground-up bark, and one-third something mineral-y like sand or perlite. I use sand if the mix needs to be heavy, to weight down the container so a top-heavy plant won't fall over.

My usual measuring scoop is a two-gallon bucket, so I mix up 6-gallon batches in a big trug or in the wheelbarrow (sometimes the wheelbarrow is occupied with one of my husband's projects).

To that mixture, I add about a half-cup of lime and one cup of greensand per 6 gallons of potting mix, and I also add some organic fertilizer. The amount of organic fertilizer depends on which one I have on hand, but you should be able to make a decent guess based on the recommended amounts for gardens.

Also, I use a little more fertilizer when the mix has soil conditioner instead of peat moss, because the ground-up wood takes more microbial "work" than the peat -- the microbes need more nutrients to thrive.

Hope this is helpful!