Sharp Practice night fighting rules

My Puebla 1863 scenario involves night fighting in a built-up area. As there aren't any rules for fighting at night in the Sharp Practice rulebook, I'm forced to come up with my own.

In the first playtest, a couple of weekends ago, I wanted to keep things simple and concentrate on the character motivation parts of the scenario. Accordingly I just had two rules:

1. All units count as one cover level better when fired at, and
2. Any unit firing removes one in every three dice.

Subsequent discussions (Mark was particularly helpful, thanks mate) led to the conviction that uncertainty as to whether a unit was going to be visible would make for a more dynamic game. With this in mind, the second playtest will use the following rules:

1. All units count as one cover level better when fired at, and
2. When activated a unit make an alertness check rolling one die as follows:

Regulars or Light Infantry                        3+

Conscripts/Volunteers or Skirmishers       4+

Militia or Irregular Skirmishers                5+

If successful, the unit receives a marker:

3D printed telescopes as spotting markers

Enemy units are spotted at the following ranges:

Line Units in the open                24" / 12"

Skirmish Troops in the open       18" / 9"

Units in cover                              12" / 6"

Units firing                                  24" / 48"

I may create some additional markers to remind us which units have fired from their current location.

If you can see the unit, you can fire at it (remembering that it counts one cover level better thanks to the darkness).