When I was younger and tried to stay much leaner than I do now here is what I would do with my ground beef. Cook it in a pan without adding any additional oil and right after it was cooked thoroughly I would put it in a strainer and run hot water through it.
I measured numerous different samples of ground beef with different fat contents and found following that approach I could cut the fat content in half. If the uncooked ground beef was 90/10 the cooked/strained version would be 95/5 or 80/20 would be 90/10. This is how I did the majority of my ground that I planned on using in tacos or spaghetti.
Hope that helps.