IF formula (cumulative)

Hi

I am failing to write a IF formula which will inform of impact of a bottleneck / constraint - some help would be greatly appreciated

Row 7 - Before bottleneck
This represents the amount of items I want to process

Row 6 - Bottleneck
This represent the amount of items that can be processed (constraint)

Row 5 - my formula
This is the formula I have written to reflect what can be made within the constraint but it is not working

Row 4 - after bottleneck (impact of my current formula
This is the answer being produced by my formula

Row 3 - what the answer 'should be'
This is what the answer should be

Example - cumulative
E7 the number of items I want to make, 17 (Before bottleneck)

Up to this point, B7 - D7, I have wanted to process 133 (69+28+36), but the constraint, B6 - D6 is 120 (40+40+40)
Therefore, I am carrying forward 13 from SEP (row 2 header) into OCT, meaning that I now need to process 17 (OCT) + 13 (carry fwd) = 30.
Meaning my answer in E4 should be 30 (as per 'should be' cell E3).
However, my formula is showing 17

Which means that moving into NOV, I have no carry over, so I can process all 31, same as DEC, I can process all 25.

I cannot figure out how to amend my formula (I have tried various mutations) to accommodate cumulative effect.

Can someone please help me

Many thanks, Maddy