I read it somewhere that FE distortion will be minimum if the horizon is smack at the center...
But looking at your photo with horizon in the middle, I guess theory sometimes is difficult to be put into practice.
A true photograph need not be explained, nor can it be contained in words.
buildings will showcase barrel distortion the most. landscapes can hide it better. if you make the photo level with horizon in the middle, then what is curved is the sky, and no one can say for sure if the sky is curved in the picture unless you tell them.
Nm, sorry today unable to check the ballhead for you at cp.