Lots of good advice here, especially the necessity of discovering if your boat can reach its rated rpm at WOT. That's the key.
However, don't get too Obsessive/Compulsive about it. When I talked with Brian Smith at American Diesel about prop selection for my Grand Banks 32 (similar to your boat), he said that these Ford diesels weren't bothered much by overpropping and that being a couple hundred rpm below the optimum wasn't anything to worry about. They aren't high-strung, turbocharged machines, after all.
My SNOWBIRD has a 24x18 LH prop with a 2001 American Diesel rated at 139hp, the final version of the Ford Lehman. 1725-1750 rpm gives just over 7 knots while 1600 rpm is a slower "Long Range" speed, which is just what the calculations predict. You can calculate performance yourself by multiplying various Speed/Length ratios by the square root of the LWL. Combining that with measured results over 1500 miles, I got:
Displacement hull speed 7.41 knots (8.53 statute mph) @ 1.34 S/L ratio
9.0 knots (10.36 smph) @ 1.63 S/L = 2600 rpm (WOT)
7.4 knots (8.27 smph) @ 1.34 S/L = 1800 rpm
6.91 knots (7.95 smph) @ 1.25 S/L = 1700 rpm
6.64 knots (7.64 smph) @ 1.2 S/L = 1600 rpm, 1.6 gallons/hr measured
6.36 knots (7.32 smph) @ 1.15 S/L = 1550 rpm
6.08 knots (6.99 smph) @ 1.1 S/L
5.8 knots (6.67 smph) @ 1.05 S/L
5.53 knots (6.36 smph) @ 1.0 S/L
Oops, you've asked the time and I told you how to build a watch. I hope this helps.
-- Tom