how do i get the credit rate % for the mortgage interest credit certificate.

This should be on the mortgage credit certificate (MCC) from your state or local government that you would have received at the time of the purchase of your home. The MCC  certificate is  issued as part of state and local government programs to help people to buy a home. 

The credit rate is the percentage used to calculate the mortgage interest credit, and is between 10% and 50% of the qualified mortgage interest you pay. The government agency who issues your MCC determines your mortgage credit rate.

The Mortgage Credit Certificate (MCC) Program offers qualified first-time homebuyers a Federal income tax credit. The credit can reduce potential Federal income tax liability, creating additional net spendable income for qualified first-time homebuyers to possibly use toward their monthly mortgage payments.