What is the weapon damage range formula?

When a gun is firing past the "effective damage range," what is the formula to calculate how much damage it does? For example a Thompson does 38 damage and has an effective range of 90 meters, when I am shooting someone at 100 meters, how much damage would I do? What is the formula for that? I am attempting to make an Excel sheet on weapon damage and how much they do past the effective damage range for each gun, so whoever can help me with this I would greatly appreciate it.