Calc: How to carry out a cumulative addition using iteration

Found the above title to a question in FAQs.
The example is given in the screenshot below.

/Users/sights/Desktop/Screen Shot 2018-05-24 at 6.16.39 pm.jpg

This example doesn’t work as I think there is an error in the formula in referencing the cell E7…


This formula is in cell E7, and it tries to add itself to the equation.
I think the cell reference should be C7, and when I add the same formula to my sheet, I get a cumulative addition, but a very erratic results.
The first calculation doubles the answer, as the FAQ explains, the next two or three calcs work fine, then the erratic behaviour starts and the cumulative result is all over the place.
Also, if I add the same number to the target cells, the formula does nothing - only if the numbers are different to what is already in the cell.
I have tried many variations of this formula without success. That said, I am a novice at spreadsheets, and my maths is only average, so my understanding of formula construction and syntax is limited.

Any help would be very appreciative.
An example of my sheets are below…

/Users/sights/Desktop/Sheet 1.jpg

/Users/sights/Desktop/Sheet 2.jpg

Sheet 1 adds a customer’s order, while on sheet 2, column C is the running total, and where I want the cumulative addition.
I need to be able to keep an accurate total that can be updated once a week, using the Today() function in A2 on either sheet.
I am in desperate need of help, so all efforts would be gratefully appreciated.

hmm… something with my video driver. The images are not clear.