The old rule of flaps is the first 20 degrees adds lift after that its just drag. This is why on a fowler flapped airplane you can use upto 20 degrees of flap on takeoff. On a split flap airplane such as a P40, the flaps do add lift but generate more drag than anything else which helps steepen your approach and allows you to make a nice stable power on landing. If you have a heavy slippery airplane you will see the most benefit. light wing loading with alot drag ie most sport planes we fly really don't benefit much from flaps.