Scandinavian Homes Ace of Spacing has to be my favorite! I'm now re-thinking what to do with mine! I have some color photos that i just might re print B&W!
Olldroo... Ditto for me! I drew round my pictures onto newspaper, cut them out accurately then blu-tacked them to the wall. I could then adjust the sheets till my 'pictures' were in the right place. I then measured the distance from the top of each picture to the top of the string when taut and marked that position on the sheet on the wall. I could make a bradawl indent through the paper as a marker. Hope this helps! Good luck.