Protection calculation

[krmiller]

Firing unit modifiers affect the Fire Value calculation, these modifiers are applied separately, not added together. This can be seen in the Fire Report if On Map Results is not selected and FOW is not in use.
The normal fire value for a 250-man unit armed with rifled muskets at one hex would be 250x4 = 1000.
A Disrupted unit would have a fire value of 500 (1000x0.50), a unit that was both Disrupted and moved would have a fire value of 250 (1000x0.50x0.50).

Target Unit modifiers affect the casualty calculation. They are cumulative, being added together then applied.
Again, this can be seen in the Fire Reportif On Map Results is not selected. Modifiers of -40-50+10 would result in a net
modifier of -80 being displayed in the Fire Report and used in the calculations.

Terrain modifiers are Target Modifiers.
Forest is hex terrain so any fire into the hex is modified, most games it is 40% but may be different.  Hex terrain does not effect melee.

Stonewall is a hexside terrain, it effects fire & melee through the hexside only.

So if the fire passes through the hexside then yes they would be added together then applied to the casualty calculatuion.