I would go with the worst of them. Thematically, if you shoot a gun in difficult surrounding, the worst of the obstacles along the whole route to the target would be the factor that make it the hardest. Or maybe you should sum them, as when shooting through multiple obstacles?
We aren't talking human eye here but cyber enhanced bodies and weapons, so just use the Targets TAC as I believe (and the rules hint at it) the TAC represents the cover/environment of the target room.
I have highlighted the relevant sentence :-
"Tactical Situation (TAC) – Every room on the board has its own tactical situation modifier that negatively affects Ranged Attack tests in the Real World phase. Simply subtract the modifier when making the test for a Ranged Attack. If you control the Terminal controlling the room in which the attack is being resolved then the TAC value is 0. This is due to the accumulation of objects that can serve as cover, as well as the amount of light. It is assumed that there is a camera security system in every room and hallway. The character that controls the Terminal also controls the associated cameras and thus gains an advantage as he knows exactly where the enemy is located and can communicate this to his Team over their Tactical Network. "