When sighting and measuring for a painting or drawing, there are two essential tools to have on hand: a straight skewer and a plumb line. Skewers are easy to come by, as one need only utilize a thin metal knitting needle or straight bamboo stick; plumb lines can be constructed with two simple elements: a thin length (a little less than 1') of dark thread and a weight.
Here, I have a metal object that I found laying around the house. Others may use a fishing weight, an earring, or even a small metal nut-- in truth, anything that will adequately weigh down your thread will do just fine.
With these two items on hand, all you need to do is securely tie your length of thread to the weight.
Et voila! It's that simple. You now have a weighted vertical plumb for sighting and measuring your drawing or painting subject.