Also - take a look at your fabric to see if it has "stripes" - many fabrics have horizontal repeats as well as vertical repeats. These "stripes" can be very subtle, but many fabrics have them. If I am using a printed fabric for a border, I like to have the "same" look on all four sides of the top.
I generally try to avoid ripping, too. Exception is when I want a chunk off a long piece, and I know I have "lots" of the fabric. If you do want to rip - do a test to see how badly tearing traumatizes your fabric. Sometimes it is more than an inch on each side of the rip.
My method is very similar to Feather3's - measure 7 inches in from the edge - make a mark - and then connect the marks using a long straight edge and pencil. I rotary cut - usually only two layers at a time.
Last edited by bearisgray; 10-06-2015 at 04:36 AM.