We tell people not to give insulin unless BG is over 200 when they are new and have little data. You have some data and giving the 1.5u dose tonight should be just fine. I would however strongly suggest you test BG again tonight +2 or +3 hours post shot or at least before bed and if the BG then is lower than the BG at PMPS, feed Mittens a snack and leave food out for overnight. If numbers were to drop to 90 or less, then more monitoring tonight would be required.
It looks to me like you were getting better numbers at 2u than you are now and that your vet has directed you to increase the dose by a full unit probably based on an in office elevated BG instead of looking at the data on your SS. When you get PMPS that much lower than AMPS, it can be indicative of too high a dose or long duration which is possible but less likely.
I would caution you to always get a pre-shot test and try to get mid cycle tests whenever possible to see how low BG is going on a dose. ProZinc usually reaches peak action in the +4 to +7 post shot and you don't have any tests in that timeframe. Randomly grabbing a test in that time period will help you see what is really happening to BG on any given dose which is information necessary to determine optimum dosing.
I would also suggest that dose increases should be done in 0.25u or at most 0.5u increments. Otherwise you risk skipping over the best dose and it appears that may be the case already.