You need to measure from the part of the hull just below the drain plug.
JAZII
That is where I took the measurement from. 16 inches to the floor and 12.5 from the middle of the prop shaft to the floor. Difference is 3.5 inches
It appears that you are measuring correctly. What type prop do you have. It that a trophy? Having prop worked might help with steering torque. Hydraulic will help more, but I know you have been weighing that option.
Hopefully, others with more experience will post better advice.
Good luck
Saving my dollars for the steering. I will have hydraulic steering.