(Mathf.Floor(ConVar.Server.worldsize / 146.3f) - 1)

returned correct results:

var z = (Mathf.Floor(ConVar.Server.worldsize / 146.3f)) - Mathf.Floor((pos.z + (ConVar.Server.worldsize / 2)) / 146.3f);

z must subtract 1 from the returned value

it depends on the map size apparently. hadnt had time to dive into this sadly.

that shouldn't matter for this specific method, which by the way is nearly identical to the one created by redBDGR and Whispers88 years ago. can anyone provide a map size and seed that my answer does not work with (z must subtract 1) ?

please do submit a pull request

you've already been given the answer {z - 1}

z-1 delivers the rigth result with the current version, but is off by one with z-1 at map size 3500.
i'm too busy to test it with different map sizes right now.
as i said: please do submit a pull request.

map size of 3500 using what seed? 

no seed, custom map. seed should not have any effect though.