I'm trying to calculate a total (in this case of a quantity of spares) from a subform. The total quantity which will in fact be a running total resides on the main form. I've been trying for days and I think I'm developing a nervous tick......
If it is that you have a column on a subform for which you want to total and display the total on the main form you can try the following:
Create an unbound field on the subform. In the control source for this field type =Sum(fieldname) (This holds the total amount). Then on the main form create another unbound field and in the record source type
=[subformname].Form!fieldname (fieldname here being the name of the field on the subform that holds the total).
If this is not what you want, probably you can give a little more details.