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Jmayargentic 

Your original Formula error in red:

=IF(SUM(BG$4:BG13)-BG$3<0,0,IF(SUM(BG$4:BG13)-BG$3>BG13,IF(BG13-BG$1<0,0,BG13-BG$1),IF(BG$1=0,0,SUM(BG13:BG$17))-BG$1-BG$3+SUM(BG$4:BG12)))

I followed it where it wanted to go:

So I entered the formula:

=IF(SUM(BG$4:BG13)-BG$3<0,1,2)

it picked 2.

then I added the next 2nd question in the formula:

=IF(SUM(BG$4:BG13)-BG$3<0,
0,
IF(SUM(BG$4:BG13)-BG$3>BG13,1,2))

it picked 2.

then I added the next 3rth question in the formula:

=IF(SUM(BG$4:BG13)-BG$3<0,
      0,
      IF(SUM(BG$4:BG13)-BG$3>BG13,
          1,
          IF(BG$1=0,
              0,
              SUM(BG13:BG$17)-BG$1-BG$3+SUM(BG$4:BG12)
          )
      )
)

It picked 1=0

The red #1 is the next 3rd question in the formula is this one:

IF(
    BG13-BG$1<0,
    0,
    BG13-BG$1
),

In reality, since the 3rd and 4th question are the same level, I would call them 2-a, and 2-b

Meaning this:

you should build your nested if statements like: IF(a=b,1,2) and go on from there building what 1 is and what 2 is so you won't get confused with the nesting and the answer the if statement returns gives you a good idea on which question you're going to ask on either 1 or 2.

cheers