Why turret complexity is a rule of thumb, not a formula

Can the increasing problems of multi-gun turrets be expressed as an exact mathematical rule?

No; the analogy is a loose guide because gun independence, ammunition supply, breeches, and layout all alter the result.

Independent elevation, shared or separate ammunition paths, breech layout, and gun orientation all change the engineering burden. A precise equation would have to model every such choice and would quickly stop being useful to a general audience. Each effort to make the rule exact introduces another design variable that can change the result. The analogy is meant to communicate nonlinear complexity, not predict a mounting's reliability.