So someone with standard vision will be able to read 50cm registration letters at about 360m or 1200' in ideal conditions.
And if you score 6/5 or 6/4 you should be able to read them even higher.
So your formation may have been quite legal
Perfectly legal even below 500ft - a bit harder of course to keep clear of all the bits you need to,especially in formation so for that reason above 500ft much better, but no LEGAL reason you need to be higher - any considerate formation would be higher though as it tends to be noisy.
