It seems like no matter how carefully you measure, the screws are always out of alignment, and the piece of equipment won't fit onto the screws.
Here's a really stupidly-simple technique I use all the time, and we use at TechShop locations to mount all sorts of equipment.
This is one of those tricks that you should really memorize and make as part of your lifelong mental toolkit.
Step 1: What You'll Need
o Tape (I prefer to use blue painter's tape because it doesn't stick too tenaciously to the surfaces)
o Pencil or pen
o Piece of equipment to be mounted
o Screws (must me the right size to fit in the slots on the equipment)
o Screwdriver to fit the mounting screws
o 2 minutes of time