Keeping percentages at or below 100

I have a spreadsheet that performs some financial mortgage calculations. This sheet has 2 sets of fields where you can enter a dollar amount and a corresponding percentage is calculated (using a constant value for calculations), OR you could enter the percentage and the corresponding dollar amount is calculated. The problem I now have is that it’s possible to enter two percentages, or dollar amounts, where the total of the 2 sets of numbers exceed 100% of the constant value.

How can I force the percentage (and therefore the dollar amount) of the other set of numbers to change if I enter a percentage (or dollar amount) that causes the total to exceed 100%. The change should cause the total percentage to be 100%.

The fields that I’m in need of fixing are the Down Payment and Loan Amount fields.

A sample worksheet is attached.