If the layout I am working on is done in straight lines and I need to measure the placement of an element or space elements evenly along a line I use my acrylic ruler. Instead of marking on the paper, lay your ruler along the line you want, apply adhesive to your element, and butt it up against the ruler as you lay it down. With this technique your element will always be square and evenly spaced!
p.s. also helps mat photos center