I want to implement general functionality (like scrollTo, style calculations, etc..) but not sure how to do it with Polymer.
I'm thinking of implementing it as a behavior, what do you think?
I think if it works in a behavior it's the right place. Here are a list of things that the Polymer team implemented as behavior https://elements.polymer-project.org/browse?q=behavior resizable, fit, overlay, sounds similar to your examples to me.