I have a collection called 'ItemCollection' with fields like itemid, ItemLabourTime (this is the time required to manufacture an item), Labour(i.e TotalLabour, based on quantity entered) and several other fields.
I'm trying to read in the value for quantity from the user in a gallery(see screenshot below) and auto-generate the field Labour based on the value entered by the user.

So if a item has ItemLabourTime of 10 mins and the user enters 2 for quantity, the labour value (i.e TotalLabour) for that item in the gallery should be 20.
I was trying to use the following formula to patch a default value for Labour to the collection -
Patch(ItemCollection, ThisItem,{Labour: ThisItem.ItemLabourTime*ThisItem.quantity})
but that was giving me an error- 'behavior function in a non behavior property'.
The plan was to set this as default and use an update in the Onchange value if the user wants to reset the auto generated value.
How can I achieve this functionality? Any help is much appreciated.