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
Step 2: Put Tape Over the Mounting Holes on the Piece of Equipment
Try to line up the tape so it is straight...you'll need it to be straight so you can use the edges to align the mounting holes to the wall or surface to which the equipment will be mounted.
Step 3: Punch Through the Tape Into the Mounting Holes
Usually I poke a hole in the main screw hole where the screw will first go into the equipment, then I move the pencil up into the track where the screw will slide up to achieve the mount.
Peel off the tape after you have poked the holes. Be careful not to tear the tape around the holes that you poked through the tape.
Step 4: Stick the Tape on the Wall Where the Equipment Will Be Mounted
Align the top edge of the tape to make sure it is straight. You might even want to use a level to make sure it is horizontal or vertical, etc.
Step 5: Screw the Mounting Screws Into the Wall Using the Tape As a Guide
Screw them in far enough to grab into the surface, but adjust them out far enough so the equipment can slide over the screw heads.
Step 6: Remove the Tape From the Wall
Step 7: Slide the Piece of Equipment Onto the Screws
If the equipment doesn't want to slide down onto the screws, unscrew the screws a little bit. Keep adjusting until you get a perfect fit.
You are done!