It is not unusual for a small, slippery homebuilt to exceed the Geometric Pitch by 10% to 15%, this is not being more than 100% efficient, it has to do with how the blade angles are measured and the fact that this formula doesn't take into account the lift of the blade airfoil.

Once you establish the maximum level RPM possible, reducing that RPM by 10% will approximate a 75% power setting.