As a physio I see quite a few SUP paddlers with shoulder pain. This often stems from punching forward with the top hand, rather than pushing down. The stroke is more like pole vaulting, engaging the blade deep and vaulting the board forward (not pulling the blade through the water - which will cavitate - if you are that strong!). Punching forward under load leads to protraction and elevation of the shoulder blade, frequency of this movement can result in an impingement syndrome - usually a sub acromial bursitis or supraspinatus tendinopathy (rotator cuff injury). Controlling the top hand over the catch and 'spearing the fish', engages the ball (humerus) in its socket prior to the load (catch) and allows the top arm then to act as a connector rod pushing down as the bottom arm assists in driving down and aft. Paddle length plays an important role in this.
Too long and you cannot avoid punching forward, too short and you will bend forward at your back. My method that I use and get my patients to use is shaft length. The blade on any paddle should be completely covered as that is the bit that is securing the water for you - this applies in any water sport. This limits the effectiveness of the overhead height measurement method, as this doesn't take into account blade length and the thickness of your board. You could be going too deep or not deep enough.
I will get a paddler on their board and get them to hold their paddle vertically alongside their body with the tip of the handle level with the shoulder (the acromion - for the anatomically minded

). The blade will be in the water in the feathered position so it lies alongside the rail of the board. The gusset or collar of the paddle is the bit that the shaft slots into, this is usually ~5cm long. For racing/paddling the water line should be just below the top of the collar. For surfing it shouldn't be any lower than the bottom of the collar (start of the blade).
Out of interest I did a measurement of a group of experienced paddlers that were happy with their paddle length after years of playing and trimming. It was interesting to find all of them measured to this point (level or just below the top of the collar). It works well for my patients. Just another method to throw in the mix.