At sufficiently high level of force modelling, it doesn't matter if you could physically shoot a thing you don't see if you don't do that because you don't know where to shoot.
In ZK, cloak (including submersion against non-sonar-equipped enemies) mainly provides the two following high-level abilities (unless disrupted e.g. by flea screens):
- First strike - a cloaked unit has the initiative, so it is always the one who shoots first in a fight. This is what matters in artillery duels where one or both parties are cloaked.
- Ignore Range - a cloaked unit can't be shot by an enemy until its position is compromised in some way, so it can attack against longer-ranged adversaries at its own comfortable range, either defensively or offensively. This is what makes Scythes and Widows so terrifying, and why Lances lose to cloaked Grizzlies.
So the power of cloak is not a constant multiplier here, i think. It's more akin to an ability to shift between linear and square lanchester laws. If your enemy is entirely Scythes and you're not disrupting their cloak, you will always be fighting in linear, at point blank, and they'll always strike first. If you manage to deny them this ability, they're stuck fighting in quadratic mode against longer-ranged adversaries with equal initiative.
If you model it like transitioning between square and linear rules, then the power multiplier for army of size X that you seek is (X-1). However, this doesn't take into account that larger armies are easier to partially decloak, smaller ones are easier to hide, and at sufficient scales the decloaked front of your fully cloaked army will be huge enough for enemy ranged weapons to work and the initiative difference to not matter. You can pick some power law for that yourself i guess.