It depends on what your BG levels aims are really - Metformin doesn't stop your BG level rising if you eat more carbs than you can cope with. To get normal number my upper limit was 50 gm of carbs per day, and I tested after meals to be sure I did not exceed 8 mmol/l - the actual number gradually fell as I got better. It is more usual to test two hours after starting to eat, - it is just what people tend to do to make things standardized.
Low levels shouldn't be a problem, as Metformin isn't that sort of a treatment.