I've tested all three of my Canon zooms, the 15.7-47, 30-105 and 30-300 with a tape. Bare in mind I don't have any resolution charts or a Siemans star, although I am going to order one. I would run the tape, set the focus and then look at monitor. I always had to make a "very" slight tweak to the lens and the peaking would pop right up. So I'm exactly certain how exact the marking are on the barrel. Maybe this is why some of the Angenieux and Fuji Premier zooms are more than double the price of the Canon's. The 30-300 is dead on at close distances. Interestingly, the closest mark is 5ft, but the barrel turns a bit more closer, just like it turns a bit beyond infinity. I went out to 28", the furthest I can get in my house, and it was almost dead on, again for a very minor tweak. The 15.7-47 I found to be out by .5" at anything 3ft. It's hard to tell anything else with the lens because it's fairly wide. The 30-105 was out by roughly an inch at 8ft. None of the lenses seem to lose focus when I zoomed out. But until I get a chart it's hard to say.
Same thing here Robyn. I back off slighly from infinity and everything snaps into focus. I'll use focus mag on the viewfinder at the same time just to make sure.