That's the million dollar question. A lot of researchers believe the Am. Diabetes Assoc. ranges are not good. What you have to do to really test it well is eat or drink pure carbs...no fat and no protein. If you test at the 2 hour mark after your last bite, here's what the ADA says:

Normal: less than 140 mg/dl
Prediabetes: 140 mg/dl to 199 mg/dl
Diabetes: 200 mg/dl or higher

Many researchers believe if your glucose spikes above 140 at any point in time after eating carbs, you have a problem. Others say normal people would never go about 120.

Here's an article you might want to read.

Why Your “Normal” Blood Sugar Isn’t Normal (Part 2) | Chris Kresser


All I know is my glucose spiking to the 300 range is not good. That's what happened when I ate a bunch of pretzels on an empty stomach.