The thing is however that the TSO-540 is connected the a constant speed propeller in general and therefore when you lean past best power to best economy, the power will reduce and the blade angle will decrease to keep 2400 Rpm
Even with a constant speed prop it matters not whether the 65% is being produced at best power or best economy. The prop will be at the same pitch since you are putting out the same horse power. Lets look at it another way. With a fixed pitch prop you settle into the cruise, full rich mixture, and set 2.400 RPM which the book tells you is 65% power. You then begin to lean and the RPM will start to increase and will reach a peak which will represent approx 68% power. This is the best power point. Continuing to lean to best economy the RPM will fall until a power output of about 64% is being produced. That 1% loss of power will show as a slightly reduced RPM (a bit less than the original 2,400) and loss of a couple of knots airspeed. I'll let you do the maths on your particular aircraft as to the airspeed change with a 1% power reduction.
