You can also use an IF THEN ELSE statement to populate month 1 for Prior Volumes
=IF PREVIOUS(Volumes) <> 0 THEN PREVIOUS(Volumes) ELSE Volumes
This makes the assumption that this is the first month so closing volumes equal closing volumes. It Also assumes that every other month with have volumes greater than zero. If this is not the case then use @JaredDolich method of populating month 1.
Also, remove the * 100% and simply format the line item as a % in the blueprint.