I think I know this answer but want to be sure.

A few months back I had a brain fart and forgot to do my visa run for a month. I paid the fine.

Now my visa will be due 18 Mar. instead of 15 Feb.

If I wait till Mar. the original visa expiry date will be past. Will I have to return on a 30 day visa exempt? or would I be better off going out on Feb 15 and get another 3 months.

In other words having 5 months rather than 3. Or will I still be able to get another 3 months from Mar 18?

If the enter before date on your visa is February 15th that is the last day you can get a new 90 entry. After that it would be a visa exempt entry that would be 15 days at a border crossing unless you are from a G7 country.

I would make a trip out and back to get another 90 days before then.

I pretty much thought so but several years back I did nearly the same on a 60/90 tourist visa, I paid the fine and they extended it too. So I ended up with 4 months.

If the enter before date on your visa is February 15th that is the last day you can get a new 90 entry. After that it would be a visa exempt entry that would be 15 days at a border crossing unless you are from a G7 country.

I would make a trip out and back to get another 90 days before then.

Very true information quoted above: only met a guy in a similar situation and he was told the exact same advice from the Immigration Office in Mukdahan.