The 6/6 standard visual acuity implies that you can differentiate lines 1 arc minute apart. The smallest discernible character is thus 5 arc minutes.
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.