LouW...this isn't exactly true. Most common propellors have a helical pitch, meaning that the pitch is the same at any point along the blade. The ANGLE of the blade varies but the PITCH remains the same. Imagine cutting a 4 TPI (threads per inch) thread in a lathe. Cut one on a small diameter rod and the the angle the thread lays over will be very apparent. This is the equivalent of the prop pitch close to the hub. Now cut another 4TPI on a large diameter (say 12") and the thread will appear to be almost vertical, or flat. So the ANGLE the thread lays over at is equivalent to the angle you see at different points along a prop blade even though the PITCH remains the same.

So if you had a helical pitch 13" prop and cut it down to 12" then the pitch at the tips would still be the same. Some props do vary their pitch BY DESIGN towards the tip. Depending on the application they either increase or decrease towards the tip. In this case the 75% point will change as you said.