Well, that makes perfect t sense. 150 grams. Yes. If you eat 3 meals. Divide that out to 50 per meal. That would be spot on. And if you do 3 meals and 2 snacks, drop it to about 35-40 per meal and 15 per snack.

The basic guide is whatever is needed to keep your blood sugar in a very good range. If whatever you are eating makes it high, then adjust your carbs or portion until you get a good blood sugar reading 2 hours afterwards. If what you are eating makes it low, then adjust upwards either portion or carbs, until you get results that are satisfactory. The doctor's guesstimate what might suit you, but they cana't see what happens when you eat their suggested amounts. Only you can tell that with your meter. Once you know which foods you can eat, then you don't have to keep testing so much. That is what a glucometer is for, although a doctor will never tell you this.

Yep eat to your meter is the best advice.
Personally I try and keep under 30g per day if not lower and eat lots of good fat like avocado, butter and olive oil.
Remember carbs turn to sugars in your body. Once you go low on carbs you end up usually in ketosis which means your body starts to utilize fat for fuel.
This can result in weight loss for some

I think this is a question that we each have to find our personal answer for. I was taught to "eat to the meter" and so I've spent a lot of time testing myself after eating foods to see just what they did to my blood sugar. I test one hour after I take my first bite and then again an hour after that. Record that food and then I know if I can eat it without making my BS spike. Many say they can eat oatmeal, I cannot. Many say to leave potatoes alone, I've learned that I can have potatoes in normal quantities and my BS remains good. This is what makes me believe that there should be no "set standards", not from the ADA or anyone else.

I was told to keep my carbs at 15-20g per snack and 20-30 per meal