Metformin takes about 20 mins to dissolve in the stomach. so I take mine about half an hour before my main meal(s). I did keep a log of bgl and noted that for me there was a noticeable but small decrease in bgl when I got it right. It needs to dissolve so it can work to block the conversion of starches into glucose in the gut so like a Tampax, it needs to expand to max fluffiness before the meal passes through.
Apart from that I believe the Metfartin takes a few weeks to build up a decent plasma level which is not so time senstive. It is this plasma component that is said to reduce Insulin Resistance, and to provide the other benefits wrt CVE protection that Met is also said to have. Metformin has 3 basic mechanisms that work to help reduce diabetes. but actually it is p*** poor at reducing the bgl levels by much, and is only good for a couple of mmol/l off the meter readings with a following wind,( which most of us Met users tend to suffer from)