Below is a picture of the S13 CA18DET engine. The blue box highlights the position of the CAS.
To measure the ignition timing you will need to connect a timing gun to either an HT lead placed between the no. 1 cylinder coil pack and the spark plug, shown below.
Or clip the gun to the loop at the back of the engine, shown below. Note: In my experience clipping onto the loop at the back of the engine can give an incorrect reading. To fix this either use the HT lead method above (recommended) or hold the timing gun clip open while taking the reading – in my experience this works with my gun.
You need to set the ignition timing to 14-15 deg BTDC (Before Top Dead Centre). The picture below shows what the crank pulley marks mean: from the left -5,0,5,10,15,20
Point the timing gun at the pulley wheel and measure the ignition timing. If you need to adjust the timing move the CAS a very small amount (~1mm) at a time until the desired timing is shown.