right click on the value and go for "Quick table calculation" -> "Percent Difference"
Create a calculated field as like below
Percent = ([July]-[June])/[June]
Right click Percent and go for format and change it to percentage.
Actually, both june and july belong to the field 'Month'. Translated, there's no field named june or july. So, it's illegally to use items like [July] in caculated filed.