Optimum paddle length is a function of torso, seat height and canoe load. For maximum control, you want your blade to submerge almost entirely (i.e., where it begins to narrow abruptly into the shaft, but no more). To get the proper paddle length for proper submerging, you need to sit in your canoe, on the water, with the approximate load (people and duffle) you intend to carry. If in those conditions your paddle doesn't submerge enough, you need to lenthen the shaft; if too much, shorten the shaft. If you intend to kneel in the center and paddle solo, you'll need a shorter shaft (2" or more) than if you're sitting the stern seat. A stern paddle tends to have a longer shaft because many canoes (particularly guides) attach the stern seat right under the gunwales. Bow seats are hung lower (closer to the water) and, thus, commonly require shorter shafts.
Measuring paddle length while standing up (to the nose or eyes, take your pick) is at best a rough approximation and at worst totally misleading. That's because it combines (and thus confuses) shaft and blade. The former is key to proper paddle sizing, the latter irrelevant (although important for paddle power). Better to use the exact conditions in which you plan to use your paddle.
It's not uncommon to have a number of paddles of different shaft lengths for different boats and/or paddling conditions.