The source later gives the row rule:
• one male follower stands on the Imam's right, level in the basic row arrangement while keeping his heel from being ahead of the Imam's heel;
• two or more followers stand behind the Imam.
So the earlier wording 'slightly behind on the right' is replaced with the source's heel-based rule: equal heel alignment is valid, but the follower must not be ahead.
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Congregational Prayer: Status, Minimum, and Basic Formation
The source later gives the row rule:
• one male follower stands on the Imam's right, level in the basic row arrangement while keeping his heel from being ahead of the Imam's heel;
• two or more followers stand behind the Imam.
So the earlier wording 'slightly behind on the right' is replaced with the source's heel-based rule: equal heel alignment is valid, but the follower must not be ahead.