I have never used it - not diabetic.
I don't know on the timing.
You do realise that the amounts required BASICALLY follows carb intake and level of insulin resistance ( there may be other variables too).
Reducing carb intake (and some increase in fat intake) along with use if glucose meter to monitor blood glucose levels will actually probably require reducing Metformin - in some cases total removal of the drug over time as insulin sensitivity and blood glucose may return to normal levels.
Of course there may be some cases that this does not apply.