Spike times differ after eating and other stuff like stress, pain and even other medications can spike you, so what is useful once in a while is to occasional figure out how your body is functioning for a couple days on the same low carb diet by
1. testing while still in bed before rising
2. testing before and in half hour intervals before meals up to the two hour mark and right before bedtime
3. take the metformin halfway during breakfast, halfway during lunch , halfway during dinner and right after a small before bed snack
4. look at the difference between #1 and #3
Once you know your own body's reaction to general diet, you can do spot tests. If your normal spike are at the 1 hour and a half mark, then that's when you test a particular food addition
benchmark low carb breafast is unprocessed meat or fish and eggs, ex. bacon and scrambled eggs. No cereal, no toast, coffee with heavy cream is cool