Chances are the print doesn't exactly follow the grain line of the fabric, in which case you will be cutting it on print/off grain in order to position the print the way you want in the border's width. Using a chalk marker like the one Clover makes, I'd first take a ruler and mark all the way from top to bottom to get a sense of where you will end up cutting.
The other consideration is whether you will need to cut parallel to the selvage or across the width of fabric. I've found it much easier to not piece borders with directional fabric, but understand this isn't always possible.