Does your ruler have a 45 degree line? If not, I would suggest getting one that does.
What I do, is I put the 45 deg line on one of the hour glass lines (where the diff fabrics meet) and then whatever I do, I keep the 45 deg line lined up.
So now, if I have an hour glass that is 5.5" and I need it to be 5" (knowing from the pattern), I move the ruler so that the top edge of the ruler measures 5", keeping the 45 deg line along the different fabrics. As long as you stay with the 45 deg line, it will make it so you have equal fabric on both sides of the center. So I get the ruler where the edge is where the fabrics on the right meet, and the 5" marking is where the fabrics on the left meet. I then cut 2 sides (holding the ruler really still). Flip and repeat to cut the other 2 sides.
Does that help?