There's no one "right" answer here. Some cats are far more carb sensitive than others. Jill's Alex can get a 30 point rise from eating a piece of cooked chicken. That's a carb sensitive cat! For most of the rest of us, above 17% is usually considered high carb.
To be honest, I don't generally use gravy-based food. Gabby can get diarrhea from the gluten in high carb food. I use either Karo or honey. I base the 1 - 3 drops I use on where her numbers are (i.e., how low), when it is in the cycle, and how fast she's dropping.