Making a Cardboard Reference Gauge for Half Units or Smaller.
If your syringes do not have half unit marks, you may approximate a measurement reference using a compass, a ruler, a fineline pencil, and some white cardboard (for sturdiness). This is most useful for U-40 syringes.
Draw a line equal to the length of 1 unit based on the syringe you are using. (If you change syringes, you may need to repeat this process.)
Using the compass, position the pin at one end of the line.
Set the compass angle to draw a small circle which is slightly more than halfway across the line.
Keeping the compass at the exact same angle, position it at the other end of the line and draw another circle.
The circles should overlap; if not, make the circles slightly bigger.
Using the ruler, draw a straight line between the 2 points of intersection between the circles; this marks 1/2 unit on the original line. (This line will be perpendicular to your original line.)
Repeat the process using 1 end of the full line, and the halfway point; that will get you about 1/4 unit.
If you can repeat it again, that would get you 1/8 unit.