Why 40% in particular? 300 grams of protein per day, every day, sounds incredible (~1.5kg lean meat), is there some science explaining the effects of such amounts of protein?

My actual calorie/macro amounts vary based on whether I am trying to bulk/cut/maintain. Two weeks ago I just finished a cut where I was losing 1.5 lbs/week eating 2200 calories/day and I was at 40% protein. I lost 25 lbs during that time - although probably 80% of the weight I lost was fat, I also lost some muscle and a lot of strength. I imagine my strength loss would have been even worse if my protein wasn't that high.

So right now I'm trying to find my calorie level where I can get back to gaining muscle and this time I'm going to try to gain as little fat as possible. This is extremely hard to do - if you are eating just too little you'll be wasting workouts, and if you are eating just too much you'll find yourself putting on a lot of fat (and then being forced to lose 25 lbs).

So I want to stay as clean as possible while finding my target calorie level. Once I find that level in the next couple of weeks I will dial it back down to 30% protein.

For someone who is bulking, the "street science" says 1 gram of protein per pound of lean body weight is sufficient. So unless the parent weighs 400 lbs, 300 grams of protein is a gross overkill. Your body cannot metabolize that much protein.

Of course, some people are in the "better to have too much protein than too little" camp, which I guess works. But it's not a requirement by any means.