Have you heard of the term for hanging work "on center"? It's a standard height of hanging art that is used by many galleries. If all the works in the exhibition are hung with their center at this height it will promote a sense of continuity even if size and shape varies quite a bit! To be clear about this method - it means that the middle of the picture is at 57" up the wall from the floor. Why 57"? It has been long used as the average eye-height.
First you need to measure from the floor- straight up the wall, use a level if you need some help making sure it's straight. Mark the wall at 57" height.
Now for a tiny bit of mathmatics (don't get cold feet):
A. Next you need to measure the artwork's height and divide it by 2.
B. Then measure from the top of the picture to the wire when the wire is held tight.
This last measurement needs to be subtracted from the first like so:
A minus B = C
Add amount C above the 57" mark and make another mark on the wall. Voila!