I test all lenses at both 25x and 50x the focal length, and I use the standard FoCal target. So, for a 600mm lens that's 49 ft (15 m) and 98 ft (30 m). Since I'm using a multiple of the focal length, the target size in the image is the same with any lens (e.g. 24mm at 3.9' is the same FoV as 600mm at 98').
I suspect your problem is either having too much vibration and/or not having enough light. How stable is your setup and what was the EV reported for the test?
When I first started using FoCal, I ran tests on the main floor of my house (hardwood floors), and I had several failed tests. I moved the setup to the basement (concrete slab) and had none. Also, I use lots of light - I point three 150 W-equivalent gooseneck lamps at the target from a distance of 12-14", and my reported EVs are in the 11-12 range when I test indoors. For my 600/4 II, I ran the test outdoors in direct sunlight, and still had the 3 150 W-equivalent lamps pointed at the target, so my EVs were in the 15-16 range. That gave me shutter speeds of 1/1250 s with the f/5.6 combo with the 1.4xIII. I still haven't done the 600 II with the 2xIII testing - I need to find a vacant football field for that...