I started bolusing for Total Available Glucose (TAG, see Kate's comment for TuD link) about a year ago. That's when I went on a lowered carbohydrate diet, about 50-70 grams/day. After reading about other's experience at the TAG group I experimented with my insulin dosing.
What I do is add up the fat and protein amount in my food. Some percentage of that nutritional content is converted to glucose by the body, especially when carbs are limited. The process is called gluconeogenesis.
For example, if I plan to eat two hard boiled eggs that contain 10.6 grams of fat and 12.6 grams of protein, I add the two together ( 10.6 + 12.6 = 23.2). Only a portion of the fat and protein nutrients can be converted to glucose. That amount varies in each person. Some TAG group members reported that 0.4 is a reasonable factor to use to start your own experiment. I found that the factor, 0.35, works for me.
So I take the 23.2 grams of fat plus protein and multiply that by 0.35 and get 8.12. I then divide 8.12 by my insulin:carb ratio. In the morning I need 1 unit of insulin for every 4 grams of carbs. In this example, I use 8.12/4 = about 2.
Since fat and protein metabolize slower than carbs, it's best to deliver the fat/protein bolus over time. The TAG group reported that a maximum rate of 1.2 units/hour worked best. So I would deliver 2 units over 2 hours in an extended (square wave, combination) bolus. If I had any other carbs with my eggs I would bolus for them up front in immediate bolus.
This is what works well for me, but I encourage others that might be interested in this method to conduct their own personal testing to determine what may work for them. Your diabetes will vary!
Using this dosing regime and sticking to a low carb diet, I've been able to cut my total daily insulin in half, reduce my A1c from 6.8% to 6.1%, and markedly reduce my BG variability. The number of hypos I experience now are few.
I know that this looks complicated at first glance. Once you start to think in these terms, however, it becomes second nature.
Adopting this way of dosing insulin has had as much of a positive impact on my BG control and quality of life as using an insulin pump, the advent of rapid acting insulin, using a CGM, and eating a lowered carb diet. It may not be a good method for everyone but it's been a big deal for me. I would never go back to just bolusing for carbs.