Welcome to the reloading club.
A good feeling, and you described it like many others.
What charge? You have to ask yourself some questions.
What is the loads priority? Accuracy, velocity, or cost?
My 55 gr 223 loads are worked up to the max charge like you did.
I pick the charge that is the most accurate that is near the max load. I want velocity, but accuracy is my #1 goal.
So some loads are 1 gr below max, others half a grain under max, and some the max listed charge.
All depends on how the loads shoot.
If low cost is the goal use the lowest charge with good accuracy the functions the rifle 100% of the time.
CCI 400's will work fine with standard 223 loads like you loaded.
If you decide to come up with a 5.56 (higher pressure load), CCI 400's should not be used. Most people would use CCI 41's for 5.56 loads.