Although the Slick Contact Form is placed as a widget, the actual markup for the form is placed on the body root, and has element inline css style rules applied to it. This makes it difficult to position the form tab (or float button) relative to the themes content wrapper. With a centered design (where the main wrapper element has right and left margin set to auto), it's impossible to know beforehand, how much the form should be offset from the browser edge to align with the edge of the wrapper.
I think SCF would be more slick, if it allowed for better styling.
If there's something I've overlooked, please let me know.
For now, I have to give up using Slick Contact Form as I can't find a way to precisely positioning it relative to the main wrapper..