I didn't look at the M* video that Iceblossom posted, it may be the same technique. But here's another Jenny video where she starts with a 4-patch made from charms, sews it RST with another 4-patch (all 4 sides), then cuts it twice on the diagonal and gets 4 hourglass blocks. When she placed one on her mat, it measures exactly 6 inches.
So I'm thinking maybe experiment with scraps or paper towels or something. Try cutting them at 5.5" or 6" and see what they finish at.