I did just that, years back. I found out that in a basic pushup the resistance equals 74 % of my bodyweight. Feet elevated, the resistance rose to 78 %. In pushups, the resistance lowers as the rep progresses and the leverages change. The 74 % applies to the lowered position, nose touching the floor. In full extension (elbows locked), the resistance dropped to 70 % of BW, just as you had heard.
Donning a backpack with weight plates in it, I found out that app. 75% of the backpack weight translates into measurable resistance in pushups. To get to a BW load in doing pushups ( ie. pressing my own weight) @ 196 lbs., I had to elevate my feet and wear a 65-lb. backpack. At this point, backpack integrity and assuming the position started to become an issue, and I switched to weighted, Gironda-style dips, which is a superb chest / triceps exercise, and can be easily loaded to 100 lbs. and beyond. Having never benched, I get compliments on my massive pecs every once in a while. 