I think with a 12' frame you'll be comfortable doing a Cal King quilt. Now with the arm of 18"-24" is a difference. I don't think you can do a 12" block on point with a 18" arm without rolling back and forth. You'll have to consider if it's a nusiance or doable to you. I'll point out Charisma has both 18" and 26" and is talented to use both to complete a quilt top. I own a 22" and decided to trade in for a 26". It's all personal preference. Feel free to PM me if you have more questions.