Okay, I messed up my first post here by getting my inequality backward. (For the algebraists among us, I forgot that if you take a logarithm with a base between 0 and 1, you have to reverse the intequality.) Here's how the math works, assuming you have R=F+B focus and blank results on your initial roll.
If you use Chiraneau's ability and then reroll, you add an average of 1+0.5(R-1) hit and crit results to your total. If you reroll and then use Chiraneau's ability, you add an average of 0.5R + (1-0.75^R) hit and crit results. Since we want to know whether (use Chiraneau first) > (use Chiraneau second), we're investigating this proposition:
1+0.5(R-1) > 0.5R + (1-0.75^R)
After some algebra, that works out to
0.75^R > 0.5
or
R < log(0.5)/log(0.75).
So if R < 2.4, meaning if you have a total of 1 or 2 focus and blank results on your initial roll, you should use Chiraneau before rerolling. As Klutz says, this is because you're relatively unlikely to replace your existing focus result with a new one, and the chance of rerolling your focus into a blank is pretty high.
But if R > 2.4, meaning if you have a total of 3 or more focus and blank results on your initial roll, you should use Chiraneau after rerolling. This is as unintuitive for me as for anyone else, but I don't think it's a mistake.