If this is the first tank of fuel you have used with the new gauge I would think you would need to refuel to find out how accurate the gauge is before trying to calibrate it. I don't have Icon gauges, but my Lowrance fishfinder had fuel flow that was very accurate after it was calibrated based on actual fuel used. It told how much fuel in gallons was used and how many gallons were left-not 1/2 tank and so on.
The operator's guide that comes with the 4.3"display explains how to set up and calibrate the fuel gauge section.
"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.