If you find yourself in a situation where you have to get tools unstuck, or you are fishing a downhole assembly, a 1:1 scale drawing can really help to add perspective to what the situation is. Using Google SketchUp or other programs. draw a 1:1 scale representation and print it out – you will surely find it helpful.
Here you can see where a 4-5/8″ Bit is inside of 5-1/2″ 20.0# Casing. Notice that the bit is almost exactly the size of the casing drift. Additionally seeing the comparison of the size of the motor (3-1/8″) and Tubing (2-3/8″) against the size of the bit is helpful.
It’s also a good idea if you will be using impression blocks, to cut out the shapes (diameters) of the strings downhole so that you can match them up when you retrieve the impression block, and apply them against the face to see if there is a match.