Effect of simple flaps

What percentage will regular flaps inboard of ailerons reduce the landing speed?

A friend ordered an airplane expected to have a wing loading of 50oz per sq ft. Reference material from RCM indicates a relative minimum speed of 38 mph and a stall speed of 45 mph. Flaps are not considered in the reference material.

I am simply attempting to determine the flight envelope and landing speed prior initial flight-testing. How much will the landing speed be reduced with the use of flaps?

Thanks.

Bill