I'm not a big fan of measuring. I avoid it whenever possible, relying on the eyeballing method instead. When I need or want something to be more precise, I use my Centering Ruler.
I love it. It is so quick and easy to use. Simply put the zero near the center of your project, then make small adjustments until the edges are both the same distance from the zero. No real measuring, no making marks, no stress if the item is a nonstandard size.
Here's the finished card. The sentiment is perfectly centered.