I using a Use Item At package to make a NPC sit on any type of chair when the player sits. Only, the NPC refuses to do so. The package is running because the NPC will remove head gear, and then replace it on package start & change,. So the conditions are set ok, and should indicate other packages not interupting. Also its running from the NPC AI package selector, rather than by script. Thats Ideal because the delayed response is more natural.

Settings
Use Item At tab
Any Object > Object Type : Furniture
Location > Near Current Location. Also tried Near Reference: player, with cell unset.
Radius 300
Sit Down flag on
Allow search > Near Current Location
Radius 300

Flag tab
Unequip weapons
Enable all fallout behaviours

I tried Sandbox with furniture selected only, but it takes ages for the NPC to sit.

Most companion mods, including mine, use a 'Find' package to make the follower sit.

In the 'Find Target' Box:
'Any Object' radio button checked
'Object Type': Furniture
Object Count: 500 - but I don't think this matters

'Allow Search' checked.
In the 'Allow Search box, something to use as a center - like current location
Radius - 250-500 - or whatever works for you.

One gotcha with this is if a bed in near, the NPC may use that instead of a chair.

Now if you made a copy of a chair in the Geck, and called it MyNPCChair, then in the 'Any Object' box, change the 'Object ID' to MyNPCChair, and make sure the find radius was large enough to find it, the NPC would sit predictably all the time. Though this method would mostly be useful in areas that you created yourself, or if you spawned your chair.

Thanks! It seems Use Item At only works properly on specific referenced chairs, like the King's chair.

Find package works fine, but chalkboards are worse than beds. It takes a while for the anim to complete. I might add a condition for sleeping and hope find picks a chair next time.