Some months back I bought two rulers from Accents in Design. I have only recently had an opportunity to have a play!
Please watch my very short video. You will notice I put the ruler at the back of my hopping foot (darning foot), I found working toward myself down the border the best. In the past when I have done a piano key border I have used my walking foot for ease of accuracy, but this way there is no turning the quilt back & forth.
I applied a reasonable amount of pressure so the foot 'runs' along the edge of the ruler. I used one end of the ruler lined up with the red sashing so I knew my lines were staying straight.
I hope you enjoy & I am interested in any questions you have.