The percentage is based on the amount of money that the guidelines say it takes to raise a child per month. Who pays the insurance on them and how much? How many nights do they stay at each parents home?
Run the calculator with these figures and the incomes and daycare you mentioned and see what your stbx is supposed to be paying based on guidelines.
With the figures you posted with his income 20% would only be $360. So according to that he is paying over 20% of his income for child support. But with the figures you posted, figuring equal time/joint custody, and not adding insurance cost, $450 is the correct amount. The basic child support obligation would be $851.00 per month. This is how much the government says it takes to raise two children on your combined salaries. He has 58.06% of that obligation while you are obligated for the remaining amount 41.94%.