Usually at arms length. Put your arms up horizontally with fist closed, and you should barely be able to skim the ones next to you or in front. After a while you know the correct distance we’ll enough that you don’t really need to use arms. Afterwards, the official cert issues “adjust left” (or right), where everyone turns their heads and make sure they’re lined up perfectly.

The only exceptions were some special drill formations ns which was almost shoulder to shoulder.