Choosing the length of the SUP paddle shaft is always a problem; There seems to be many factors to consider so to help you determine the perfect length for your paddle. Here is a video of Quickblade owner Jim Terrell giving some advice on the perfect SUP paddle height, and dispelling some urban myths of paddle length measurement.
If you are unsure the 10% theory is the general setting, make sure you’re measuring the total length of the paddle from the tip of the paddle to the very outer edge of the grip/handle.
These are idealised settings, not mandatory, and obviously you have to take into account the users of the paddle, dropping 300euro on each paddle for each member of the family may not be possible. So paddling with an adjustable paddle maybe a solution to find your preferred length for all the family.