I wanted a centering rule, and decided to make one from an 18 inch steel rule I own.

Tools

P-Touch Label Maker
Scissors
Knife

Materials

Steel ruler
Black on White label tape
White on Black label tape
Clear packing tape

Step 1: Make Numerals

I am alternating black on white numerals with white on black in order to lessen confusion when using the centering rule. I am making odd number numerals black on white. I am making even number numerals white on black. I left three spaces between each numeral when setting up the label maker. Because it is a centering rule, two of each numeral will be needed, one for the left side and one for the right side of center.

Note: If you do not have a label maker, you can write numerals on a strip of paper with a marker pen and tape or glue the paper strip to the ruler, perhaps even mark them on the ruler itself. You could even print the numerals on a computer.

Step 2: Apply the Numerals

I used a knife point to position the numerals before pressing on each to adhere them to the rule. (You might want to clean the rule of any oils from your skin before applying the numerals.)

This label tape has a slit in the backing paper that runs lengthwise. Simply fold the top half on the printed side toward the bottom half and the backing paper begins to separate. Lift off whichever half you wish. I lifted off the top half and attached the numeral to the knife tip.

Notice that I cut a triangular pointer from some of the scrap tape and applied it at the mid-point of the rule for zero.

Step 3: Remove the Other Half of the Backing Paper

Once the top of each numeral adheres to the rule, flex the numeral in place so the bottom half of the backing paper peels away from the numeral. Remove what is left of the backing paper.

Step 4: Cover With a Protective Layer

I want to protect the numerals from coming off later. In addition the markings on the rule are painted rather than inscribed. I stretched a strip of clear cellophane packing tape over the length of the rule and applied it so there are no trapped air bubbles. Then I used a knife to trim the tape flush with the edges of the rule. As it is, I can continue to use the rule to measure lengths up to 18 inches, or I can use it to find the center of something.

The photo in the Introduction shows the rule in use. You can see the center of the envelope is 2 7/16 inch from either side.