...I would do this:

Turn ON the Motor. Let it ON for 1~2 seconds.
Turn ON the STROBE Signal, for a short period of time somewhere around 1 mSec. for each group.
Print for 5 to 6 lines.
Turn OFF the Motor.

Find out the result on the paper. There will be dots in the printed lines.
Next I would try to get a single dot in each group by changing the ON Time of the Trigger pulse for each group. See to it that I get single dot for each group.

Next start printing alternate 10 dots in each group.

This method is a very crude method, but has worked for me every time.

Of course, you will get expert methods from other members. Which method to use, is up to you.