Bobby, just go as far down as you can. That should be fine, and go wide then, mix in the compost mix with your native soil, and when you are done planting the clem, top dress with the compost too. Keep the clem well watered until it's established. What kind of clem?
Good Luck with your clematis!

Cathy

Wide is more important that deep unless you have poor drainage. Dig compost into the surrounding soil as wide as you can.

My Clematis that have done the best are in beds that have been completely tilled and composted rather than just plopped into native soil....The difference is huge.