why dont you did this ranged attack like artilery in realism invicus? max damage done by arrows is 5% max enemy hp, action cost one movement point for archer (unit can move or shoot ranged, unit leads by warlord could attack and move in the same turn, additional promotion) and unit can attack only one enemy per turn?
longbowmans and horse archers could do max 10% to damage, arquebuisers 15%, rifleman 20% infantry 25%, modern and mechanised infantry 35%, acv 40%
This will be unhistorical (archers could finish off an opponent as successfully as machine gunners) and makes early long-range units catastrophically useless.
second option would also increase value of "trench" promotion (additional line who reduce value of ranged attack. or maybe add additional line promotion who reduce damage from ranged attacks)
Now purely theoretical reasoning. In a good way, it is necessary (this is by no means Zeta's advice, because it is difficult)
1. to give the opportunity to build trenches - this is in one of the modifications of the TTT.
2. differentiate the damage limit not only depending on who is shooting, but also
a) depending on the target.
b) depending on the terrain and improvements, where the target is located. For example, artillery can destroy ships, other artillery and tanks, but cannot completely knock out infantry at all (early artillery) or in a city/trenches/fort (late).
A more realistic, but difficult to achieve option - there is no limit, but there is a decrease in accuracy depending on the above factors and the health of the target. "The infantry in the city can be finished off with artillery, but it will take a very long time."
It's quite good to give the shot a value. In PIE, they "shoot gold", I quietly dream of imitating a full-fledged economy and spending ammunition. In the thread Platy is a code with "calculable" and accumulated resources (like in Civilization-6).But it needs to be completed.