Your planting depth in the pot is not so important as when you plant in your garden. In a pot it will grow only a short time before fall will make it dormant in your zone. I think I would sink the pot in the garden as you don't know now where you will plant it. When it does go dormant I would cover with leaves to prevent dehydration over the winter. I would not fertilize at this time. When you do plant in the garden, prepare the area soil with compost dug into the soil. This from a peony grower in a much different climate. Al

Daniel, I'm closer to your climate, but what Calistoga said was perfect. Plant the pot in the ground, outside, and cover with a little mulch. Don't fertilize yet. Plant next spring out of the pot as soon as you can, preferably before the top growth gets very high.

Here is a link that might be useful: Garden Musings