You can get 150% return for three labourers in a T8 house for all journals up to and including tier 7, for both gatherer and crafter labourers (not in the same house of course).
Tier 8 is where you can't get a 150% return, even with max happiness items and a single labourer (I'm not sure how close you can get with a single gatherer labourer). Apparently you need a guild hall to do it, and that's expensive.
so best is to keep houses at t6 and guildhall to tier 8 ?
It's best to just not get super hung up on it. Your best value is always to have the most laborers you can with the highest happiness you can have them at. Any attempt to squeak out a little more happiness to T8 journals comes at the cost of losing out on multiple laborers per island. It's hard to justify the extra expense for the trivial boost in per laborer productivity.
I think you can crack out 12 T8 gathering laborers with 150% happiness at T8, in a guild hall, but the cost of that extra happiness is losing 3 laborers. With 15 laborers you'd be at 145%?. So, if you consider a T8 journal to be bring in about 80k@100%, then your GH with 15@145% laborers is bringing in 1.74M/day and your 12@150% will bring in 1.44M/day. So while you are earning 4k more silver per laborer, you are losing out on 300k per GH, and T8 GHs are quite pricey.
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