Invoice total calculation field - how to make it stored?

My Filemaker solution has, at its core, a fairly straightforward invoicing structure. There is an Order table, and an Order_Item table for the line items.

The Order table has an Order_Total calculation. At the moment this has a very simple definition: Sum(Order_Item::Cost)

The calculation cannot be stored as it references a related field. This arrangement seems to be affecting the speed of the database as there are lots of other calculation and summary fields which all use Order_Total and therefore, in turn, have to be unstored.

We now have ~22,000 records in the Order table and ~85,000 in Order_Item so I imagine there's a lot of recalculating going on in the background.

Is there a sensible / recommended way to make this calculation storable? I know I could use a script to insert the total, but that would mean a lot of script triggers etc. needing scattered throughout the solution. The total needs to update fairly quickly whilst staff are adding items and building an order, but after that it tends to stay unchanged apart from the occasional amendment.

I've been using auto-enter calculations for some other fields, would that be an option here?