Reduces duplication and complexity around % fields
Reduces duplication and complexity around % fields

According to codeclimate, this class was our most complex. With that in
mind I set about tidying up, and also reducing some of the duplication
around how the various percentage fields were declared/used.

moneyadviceservice/mas-rad_core,moneyadviceservice/mas-rad_core,moneyadviceservice/mas-rad_core