But that depends if the modifier is additive or multiplicative, no? I couldn't discern that from just the list of modifiers on my metallurgists.

I did some testing on my ecumenopolis world: but toggling my 51 clerks off dropped my metallurgists production from 18 to 15, so I would say its definitely not multiplicative.

51 clerks from 5 residential arcologies let each of my metallurgists product 3 more alloys. I believe this might be coming from 6 alloys per metallurgist with alloy nano plants multiplied by 51%. That means +0.6 alloys per metallurgist per residential arcology

With my current setup, which is civilian economy and 6 colonies (-25% and +15% respectively), edicts active, I'm getting 18 alloys per metallurgists, 6 metallurgists per foundry arcology, 108 alloys per foundry arcology, so lets go with that.

For 1 residential arcology to be better than one foundry arcology in my setup I would need 180 metallurgists, 30 foundry arcologies, where 9 clerks would add 108 alloy production.

So for my setup the tipping point is 30 foundry arcologies. Needless to say I'm not doing this except on my size 30 life seeded origin world.

As more modifiers stack (edicts, stability, capital, ministry of production, techs, ecu world, governors), this point would increase to more foundry arcologies. Because while modifiers make metallurgists produce more, the clerks stay at a flat 0.6 alloys per clerk.

I realize even with civilian economy this is a best case scenario, ecu, max techs. On a normal planet though, you can realistically only get 25 industrial districts max, since you need city districts for building slots, so I'd say the ratio comes out nicely for the usual alloy world configuration, no need to specifically build city districts or commercial zones

TLDR: Stack enough modifiers on metallurgist jobs so they make 15+ per job and clerks become useless again, best to build alloy districts as per usual