Four ratios:
light to dark: 4:12
dark to light: 12:4
black to white: 2:4
white to gray: 4:10
gray to white: 10:4
any of those work.
What you want to do with this equation is make a triangle connecting the two points you have and count the distance (rise/run) for this you would get 6/1 !
height or altitude for a parallelogram means the perpendicular distance from one parallel side to the other. Check the picture below.