A chef told me my ground beef was too lean and it changed how I run my case

I run a small shop in Salem and for about 6 years I pushed 85/15 on everything because that's what I was taught. A chef from a burger place downtown came in maybe 8 months ago and asked for 80/20 for his patties, and when I told him 85/15 was leaner and better he just shook his head. He told me the fat is what carries the flavor and keeps the patty from turning into a hockey puck on the flat top. I tried a batch at his ratio for myself and he was right, the difference was not small. Now I keep both on hand and ask customers what they are cooking before I sell them anything. Curious what ratios you all default to for burgers versus meatloaf, because I feel like I am still guessing on some of it.