There are dozens of ways to determine the "correct" paddle length, but they are all guidelines; the above info scratches the surface, and is all good. This is why many people have many paddles, of different lengths and shapes. I typically kneel, and like a paddle that has a shaft length (leave the blade off for now) that is the distance from about my shoulder or chin to the water's surface. Blade length is added to that. But how high I perch in any given canoe, also bow or stern of a tandem, varies, so... "I need just one more paddle"... that might sound familiar! lol
The important part is how it feels to the individual, and their style of paddling. On shallow rivers, I like a shorter blade length and overall length, than on deeper water. Try to borrow a lot of paddles at events such as Assembly, so you can try many different lengths and blade styles, before you end up with 20 or 30 paddles, most of which you don't use...