Show sub table formula result in main form
I have a sub table that shows appointments, a marked delivered checkbox and invoice value. Within the sub table I have a formula that will add up the invoice values of all delivered appointment. So, I may have 5 appointments, each with a invoice value of £10 each. The total value of appointments would be £50. If I check the box that says appointment deliver on just 2 of them, my formula result shows £20 in a sum value at the bottom of the sub table. All good so far.
Now, I want to take that formula and put it on the main form, so I see a "Value of Delivered Appointments" box.
I cannot copy the same formula from the sub table as it cannot find the data.