I'm planning to build a stone post for a garden gate entrance. From what I'm finding, it seems that the recommended approach to doing this is to start with a solid 4" slab to go under the stone column. This slab would be 18-24 inches in the ground. Is this good? The column will be about 18" square, 30" tall, and the stone will be mortared. It's something I've always wanted to try so this is my first shot. I just would hate to see it leaning one way or the other after a few months.