He says it's running between 45-4650 full out
OK, so you don't know and your making stuff up.
The reason we need to know what the GPS speed, is because this determines prop slip. A 19 pitch prop moves 19 inches forward for every revolution. The slip is calculated to determine how much less then 19 inches it is moving. There is NO such thing as a Negative slip. A Negative means the prop is move farther in the water then the picth
So if we use your provided numbers
Speed 45 to 46 mph
Ratio 1.81
Pitch 19
RPM 4400-4800 (comes from the manual)
Use 19 pitch and 46 mph at 4400 rpm = (Neg) - 5.17 percent slip
Use 19 pitch and 46 mph at 4800 rpm = 3.59 percent slip
Use 19 pitch and 45 mph at 4400 rpm = (Neg) - 2.88 percent slip
Use 19 pitch and 45 mph at 4800 rpm = 5.69 percent slip
Even if the Negative numbers are thrown out the 3 to 5 percent numbers are not realistic. I would expect to see 8 to 10 percent slip.