Down payment order of precedence

As specified:

> [DEFAULT] The DP% is 10% of HP AND the $ is the monetary value of DP%
> IF user modifies DP$, the DP% updates to reflect the % that DP$ is of HP
> IF user modifies DP%, the DP$ updates to reflect the $ that DP% is of HP
> IF the user modifies EITHER DP% or DP$, whichever is modified last becomes a CONSTANT and the other will calculate against it in relation to HP.