You have "obscured/complicated" something simple (I think)!
You want to find the maximum mortgage amount, calculated from
the down payment offered: given is down payment amount and 80%.
If down payment = 12,000, then (M = mortgage amount):
12000 = M(1 - 80/100)
.2M = 12000
M = 12000/.2 = 60000
With P = given percentage and D = given down payment, formula is:
M = D / (1 - P/100)