There are several theories. Some will prop as close to the upper end with a Light Load, others will prop closer to the Rpm where the engine is making peak HP. That can be difficult to find and it not always what the Spec says. Case in Point, for years OMC made a 90hp, a 115, and a 140, all sharing the same displacement. All were rated their respective HP at 5000 rpm and all had a WOT range of 4500 - 5500. They did not all make their Highest HP at 5000. The 90 made it's highest power at 4600-4700, the 115 was closest to the 5000 mark, and the 140 did it's best between 5350 and 5450 rpm. If propped to hit the highest HP rpm, that is where you will get the Highest top speed. Propping for either side of that Rpm will result in a slightly lower Speed as less power made. A 90 revving to 5500, is making less HP than it did at 4700
If propped to it hit the top of the Rpm Range when lightly loaded, means you or anyone else is not going to Over Rev the Motor. Propping too low, does increase the Temp of the Piston Crown, and that can cause Oil in Ring Grooves to cook and turn into Carbon, resulting in all sorts of Misery
If I were you, I would leave the 13 on it, adding pitch will also slow down the Acceleration