I'm not sure what the board says.
But the math stuff, I was trying to figure this out.
You have a total amount that somebody is invoiced for, say $10,000. The person makes a 10% deposit, which means they owe $9000. Later on, they have to make a down-payment to bring the invoice to half paid, $5000. I was trying to figure out the formula based on the Amount (A) and Balance (B), how much they have to pay to get to half of the total amount.
I should also mention, in case it wasn't obvious, that's my desk at work. I also fixed the first post to show both pictures, front and back, I used to have two links to the same picture. Oops