Hello there Richard.
Firstly, could you confirm if you are on "ordinary" Metformin, as opposed to slow release option?
Bear in mind that Metformin works on a therapeutic dose That means a dose that builds up over time, rather than a tablet by tablet basis, it probably wouldn't make a heap of difference.
If you are taking the slow release (sometimes called Extended Release = ER), then it is important that you do not half or cut tablets up in any way, so when you take your tablet may depend on which tablets you actually have in.