I'm at 10 years, and this is basically how I do it as well. The math just doesn't work, because there are too many factors at play, most of which you don't even know about, and can't plug into a simple equation. You might have the exact same meal and the exact same insulin dosage on two different days, and go low on one and high on the other.

You eventually gain an instinct for it. Doesn't always work, but for me it works better than the formulas...