Propping is a trial and error endeavor, it is not cut and dried science. Similar rigs may need different props depending on what is most important for the operator. Every propeller choice is a compromise of one sort or another.
Look at the Evinrude performance reports for a guideline and talk to your dealer and Ranger if needed. If you have the Magnum gearcase on your HO, try the 21" Rebel initially to see what kind of baseline you get.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.