It's impossible to predict accurately without a continuous blood monitoring device, unless you test every 15 minutes 24/7. Or at the very least know more or less when your peaks will be after eating. My peaks on my diet are normally about 90 minutes after first bite. You also need to know how long you are higher after a meal - in other words, how long it takes you to return to base. The area under the curve is what matters.
Personally, I know I am a high glycator, which means my HbA1c will always be higher than any predicted level no matter how often I test. It also depends on your haemoglobin levels and sizes of red blood cells. There are also low glycators whose HbA1c will always be lower than predicted. It is never an exact science.