No less then 100DPI.
Why are you using the M07 to turn the laser on? You can just swap the pin # your using for the coolant command over to the Spindle output so the M03/M05 commands can be used. Or, you can add the M07 in the pre-code.
Like I said before, you need to increase the feedrate so it does not burn too dark. I'm not sure I understand the numbers you have written down for each test. If your using Metric units, you need to set the DAC axis steps per in Motor tuning to 393.7. Set the accel the same as your X&Y axis's and use a 0 Min to Max -.6477 setting range in PicLaser. This will give you the 0-5v PWM range to the driver. You adjust the feedrate & Min/Max to get the proper shade range on your material.
The 30mm focal distance is for cutting from what I understand J-Tech's theory is, not photo engraving. At 76.2mm the focal spot is smallest a longer distance so It's a little more forgiving if your material is not quite flat or varies slightly in thickness.