Gibbs 3 gal charred keg question

[Boda Getta]

I am in the process of ordering a Gibbs 3 gal charred keg for my UJSSM and Sweet Feed white dog. I seldom will have 3 gals to age at a time; I would likely age between 1 and 2 gallons most times. Is there any rule of thumb as to the optimal level in a keg. Would there be a problem or concerns if aging as little as 1 gal in a 3 gal keg. Any thoughts/suggestions much appreciated.

BG