To take measurements inside an area like a cabinet drawer, use your measuring tape from inside edge to inside edge. Take the reading and then add on the length of the tape housing which is often stamped on the case. Another way is to use your tape along with a combination square. Position the square upside down in one corner and pull the case past the square's blade. Select a number on the blade, and add the length of the tape.
A fastener in sheet metal should not be any closer than four to six diameters from the next one. For example, if you are using a 1/8-in. Pop Rivet, the nearest another rivet should be is between 1/2 to 3/4 in. By using this rule, you will reduce the chances of damaging a part, as well as make optimum use of your fasteners.