In the old days, this worked because there were few choices of breads. Now, checking grams of carb is more precise and accurate, using food labels. In early days of exchanges, we didn’t have food labels like we have today.
Pumps also made the math easier, to count grams instead of exchanges and be more precise.
However, it wasn’t until I found online groups that suggested also taking into account protein grams into calculations. Sometimes referred to as TAG - total available glucose.
I include protein in most meals, since it slows the bg spike of carbs only.
Here is old post about “taggers”.
https://forum.tudiabetes.org/t/taking-insulin-for-protein-and-fat