As said, testing is the way to decide on your tolerance. As a Pre-D certain things affect me more than others. I've found over the last 2 years that the worst culprits for me seem to be sauces and glazes, such as barbecue sauce, sweet mustards etc and the killer are brioche buns!
I can eat a plain burger (or with cheese) with half a normal bun and a few chips with acceptable affect on blood sugar. On the occasion I have had the same with a sauce or relish my levels have sky rocketed.
Last night i was in a difficult position, train cancellations, family emergencies, larder empty, at 10pm I ended up visiting the local chippy. I only ever go there like once a year but last night i felt like i had no choice as it was so late. I had a chicken tikka kebab in a pitta bread. I have to say it was delicious, i tested at 1, 2 and 3 hours and never went above 6.5. I was expecting a bad FBG but to my surprise was 5.4 this morning.
Won't be making a habit of it though, still felt mega guilty afterwards