When I plan a seamless border, I use a 3 yard length of fabric. I open it, and hold the fabric by one selvage edge. Then I begin to fold, lining up the selvage edges. I do trim the selvage edges when I get it all lined up. If you fold it into 4 layers, you will need to use a 24 inch rulrer and a small 12 in ruler, or you can slide the ruler carefully to extend the cutting edge to the remaining few inches. But with the measurement you need to cut, this should work very well for you.
If you plan to cut the border at the beginning of your project, the remainder of the fabric can be used in other parts of the quilt. A seamless border looks so nice.
I would never rip fabric when we have mats and rotary cutters to do the job. Ripping stretches the fabric. Sure, long ago women ripped their fabric, but why add an extra step (steaming it back into shape) when you can do it just as easily this way?
June in Cincinnati