use a typed expression with a scalable item.
"id":"_ID_",
"jsonrpc":"2.0",
"method":"hub.scenes.expressions.set",
"params":{
"name": "kitchenHallTempAverage",
"params": {
"items": [
{
"name": "kitchenTemperatureSetpoint",
"_id": "5fecdf73222aaa175f6d6be4"
},
{
"name": "hallTemperatureSetpoint",
"_id": "60c7696b933ffb710a8be319"
}
]
},
"code": "return { value = (params.kitchenTemperatureSetpoint.value + params.hallTemperatureSetpoint) / 2, scale = params.hallTemperatureSetpoint.scale }",
"valueType": "temperature",
"metadata": {},
"variable": false
}
}