Perhaps you can figure out how much wiggle room you have by understanding what a1c really measures. It's an overall average of blood sugar during the prior 3 months or so. It measures how much glucose has attached itself to our hemoglobin. I'm not a physician, but as I understand it, an a1c of 7 means that 7% of our hemoglobin has glucose bound to it. Depending on the lab scale used by your doc, a normal a1c might range from 4.6-5.4
Many of us look at our 3-month a1c test as a personal diabetes report card. A good number makes us feel we've achieved a good result and a high number can make us feel as if we've failed. Truth is, an a1c result should not be much of a surprise, since it often correlates pretty well with our meter average. The meter only has results from when we decide to test, but the a1c has some results from every minute of our lives -- day & night. Nonetheless, most folks will have a higher a1c if their meter average is in the 150-200 range than if it's in the 100-150 range.
With lots of knowledge how our diet, exercise, and insulin/meds affect our BG, many diabetics can achieve a1cs below 7 fairly consistently without dangerous lows to compensate for dangerous highs. The idea is to discuss a target BG range with your doc and then strive to stay within that range. Of coruse, life gets in the way even with the best of intentions, and yet the more minutes we spend with too-high BGs and/or too-low BGs, the more we invite the devastating complications taht this disease can cause.
To encourage you, however, here's a bit of my story. I've had type1 for over 35 years and spent the first 15+ years before finger-stick bg tests were even invented. We never knew what our bg was any moment in time. Now we can know and we use the information with our docs to adjust our routine. Despite the lack of technology & info available to us "back then," I am healthy and lead an active life. Diabetes ... and diabetes management ... have never kept me from dreaming big and working hard to achieve those dreams. Do I have lows & highs still? Yes, of course. Do I have treats (sweets, pizza or fast food)? Yes ... it's not the mainstay of my diet but I have them when I want because I've learned how to manage my insulin to compensate for the treats I enjoy occasionally.
That's my best advice to you because if there are foods you really like, there's no reason to tell yourself you simply can't have them! You "just" need to test & learn.
Any time you slack off will affect the HBA1C. i don't think anyone has discovered how much slacking will do the trick. i know that going above the normal range of 120 will have an effect. Though your blood sugar is going to go there without your having done anything wrong. But anything above an 8 is considered bad.