You'll have a bit of 'fun' measuring rostral length of the lower beak in an octopod, given the lack of a clearly defined 'jaw angle'. Prepare to pull your hair out; differentiating beaks can be tremendous fun (and equally frustrating). When doing so, especially when comparing closely related taxa, keep in mind that the differences between them might not be obviously mathematical (especially if you limit yourself to a few basic measurements), but in some other less-easy-to-define character state.
Take a good look at those beaks ... throw your ruler away .... and understand how they are put together. Differences are remarkably subtle, but once familiar with beak construction they are as distinct as apples and oranges (or pears as those in the UK might say).