You did purrfect!! I know it's scary to see a number that close to the "OH NO!" number, but remember, there's a little bit of a built in safety net already. We have the "take action" number set at 50 for newly diagnosed cats (less than a year) but for long term diabetics (or cats that don't hold their reductions), we actually have them drop below 50 three times or below 40 once.....so between 40-50 is that "safety net".
If you're home and able to test, you should almost always try a little LC first. Most of the time, that's just enough to nudge them back above 50.
It can also depend some on when they drop. If it's early in the cycle and you know your cat usually drops until about +7, then you might want to give a little MC first. It's all about "Know Thy Cat".
Since this 51 was late in the cycle, feeding something MC or HC would be over-kill....you just want to nudge them, not drop kick them....LOL
If you always jump to MC or HC when your cat is nearing 50, you're never giving her the chance to earn a reduction. The whole point of all this testing and record keeping is to learn how to safely keep them in normal numbers and let them earn reductions until they're (hopefully) on zero.