How can I control postprandial blood sugar to be under 200 mg/dl

Welcome John
I don't think the c-peptide needs a known amount of glucose to be consumed at any particular time.
You can have it fasted, which will tell you how much background insulin you're producing, or after food which shows how much is produced In response to higher glucose levels. Either way the insulin level will be compared to your glucose level at the time to discover if you are making less or more than is normal, usually for us T2's it's higher than normal, unless of course your pancreas has thrown in the towel.
The only test I can think of where you need a measured amount of glucose is the Glucose tolerance test. That where they measure your blood sugar then give you a set amount of glucose, usually in a disgusting drink. They then monitor your sugar levels over a set period of time to see how high it takes you and how long it takes to come down.

I don't think it matters too much when the test is done because the lab will just compare the amount of c-peptide with the amount of sugar in the blood sample to get your results.
If you want to make sure your blood sugar is at its highest I suppose somewhere around an hour after eating. But of course that would depend on what your meal contains, not everything digests at the same time.
As to the Glucose tolerance test, it depends on what you are trying to discover with the tests. I can't see it being much use to you. It was used mainly to diagnose before they started relying on the Hba1c, now I believe an extended version is helpful in diagnosing Reactive Hypoglycemia.
I would think that after 15 years you are not questioning your diagnosis, just trying to figure out why your postprandial is so high.
If you have ruled out your diet as the culprit I would think that knowing if your pancreas is producing a decent supply of insulin would be what you need to know.

johnbalc said:

diet is partially the culprit. I need to do better. Thank you catinahat, I will schedule the tests and post the results here

Click to expand...

You can use your Libre to help you with your diet, generally speaking someone who doesn't have diabetes will see their blood sugar rise after eating and fall back to around their pre meal level within 2hrs of their first mouthful.
So we try to do the same by noting what we eat, how many carbs are in the meal and giving ourselves the same 2hrs.
Now because we're diabetic and our testing methods are not exactly 100% accurate we give ourselves a 2mmol leeway (that's 36 mg/dl)
So make a note of the time you start eating, your blood sugar level, the food you ate and the amount of carbs in that food.
2hrs later check your blood sugar levels again , now don't get too hung up on the actual numbers because it's the difference between the pre meal level and the 2hr level that's important.
If your 2hrs results is around the same or no more than 36 mg/dl higher then that meal was perfect, your body handled the carbs as it should.
If on the other hand the difference is more than 36 mg/dl then you know that meal had too many carbs for you to handle. Now you can take a look at what you ate, see where maybe you can make some substitutions to lower the carb total or perhaps a smaller portion of the carbage and add more good stuff.
It takes time and a lot of experimenting using yourself as the lab rat, eventually though you will have a diet specifically designed for you, by you.