When setting up my designs, I use one of the popular calculation tools on-line to get the ball park cg figure. It's a reasonable starting point. For example, for a typical (for me) wing area, stab area, moment arm (ac to ac) and 15% static margin, the tool often returns the "IDEAL" CG AT 32% mac. This cg location will typically push to belly in knife edge flight. I have to trim quite a bit more to minimize the push. I get close to zero mix but find my planes can use a couple % mix so I add it and go play. I don't sweat the small stuff...2-6% is very small actual movement, so it's no biggie in my book.