Hi Celast,
as Fergus says, it's about knowing what your body reacts to. A reading above 7.8 (or 8.5 officially) is probably doing you harm, so your target should be to never let your levels go that high.
So you test your blood glucose (using a meter) before you eat something. You write down what you're eating. You then test again after 1hr ideally and then after 2 hrs. Some people try to get the 2hr reading back to the same number as the pre-meal reading, but I find that rarely happens. My aim is to keep all my readings below 7.8 .
I found that impossible when I started but now, 4 months later, it's relatively easy.