As I have written before, and @JohnLawler comments, punctuation is not a question of grammar in English (unlike German, for example). Fashions in punctuation change (nineteenth century authors punctuated rather differently), so when I make a suggestion based on one style of contemporary punctuation, that is exactly what I mean. There are other styles.
So, in one style of contemporary punctuation, commas, semicolons, colons and periods/full stops indicate pauses of differing lengths in the way a sentence would be spoken. The purpose of this is to help the reader to read the sentence fluently and to understand it. In the example the poster gives, I personally would either:
- Not pause for the phrase “at least”, in which case I would omit the comma:
But at least I figured out…
or
- Pause for the phrase “at least”, in which case I would insert a comma before, as well as after, it:
But, at least, I figured out…
I would regard it is a personal choice between the two, and I would probably prefer the second. However many people (or at least my wife) would say that I over-punctuate. Read it aloud and make your own decision.