I think that's fine. You want to feel she is safe. The 77 is actually a great midcycle number. The next time you see one in that range just give her a little snack and retest in 30 minutes, as Misty said, to be sure she is rising, not falling. You really don't need to worry unless you see a number in the 50s. If that happens, do the food, retest routine until it rises. If it would fall, give some gravy off high carb food. If it would continue to fall, honey on the gums. Most often, you can steer any low numbers higher just with food.
But the first one, out of the blue, is scary. Next time you will be prepared.
Looks like she bounced right away! She should come back down. I'd continue with the one unit, as long as you are around to monitor. If you can't watch her, then maybe a tiny bit less?