Sewing gauge for even button spacing - photo

[desbromilow]

and if you're in a hurry, there is an even simpler tool, but it takes 3 hands to operate.
take a length of elastic (or a large rubber band cut to be open) and lay it out in a line next to a ruler. Mark on a number of evenly spaced marks (1" for elastic, 1cm for rubber bands) with a pen.
then take the elastic (or band) and place one mark at a reference point, and then stretch the elastic so the "n-th" mark is at the other end of the distance to be divided. Then get your third hand (pretty assistant) to mark the material at the corresponding divisions.
The method I've described works on the principle that the elastic will stretch evenly along it's length, and therefore the marks transferred from the ruler will be evenly distributed along the stretched length.