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The 172 was just after she ate? We tell brand new diabetics not to shoot under 200 but to wait 20 minutes, without feeding and retest. Lots of people do shoot under 200, but only after they have lots of data to give them an idea of what the insulin might do. I think, since you did feed, I would wait for at least 2 hours and take her blood level again. (The idea is that food raises blood sugar levels and it will take about 2 hours for the number to be not "food influenced".) If she is above 200 and it will fit into your schedule to shoot again in 12 hours, give some insulin then - maybe .5 units.
It certainly doesn't look like she needs much insulin and it is better to be cautious at first, until you see how the insulin is working.
If this really messes up your 12/12 schedule, I would wait until the normal time you were going to shoot the next dose.
Does that make sense?