Before the super rank (5th) soul panels became available, the highest possible percentage of completion for all soul boost panels was 398%. Now that the super rank panels are available, the new max is 498%. The 2% that is impossible to complete is essentially the middle white panel that for all previous ranks requires being unlocked before you can access higher ranks.
You currently can't unlock that one and achieve 500% completion because the 6th rank of soul panels is not available.
I hope that clarifies things ^_^
