Function that gives huge numbers when correct solutions correspond to small numbers

Hello,
I am fairly new to excel so maybe my problem is easy to solve but I searched for a solution and I couldn't find it, so please help.

I got a table (3 columns and 7 rows) where I have the following function to calculate the values that I need, 1 per row.

"=IF(C6=0,0,(E6-$B$15)/C6)"

the B15 cell corresponds to "=-SUM(D6:D12)"

The problem is, when I insert a value in the "C" column excel exponentially increases the value for the function I mentioned earlier... this means that I got huge numbers when in fact small ones would be the correct ones.

I enabled limited iterations because there are circular functions.

You can find the table attached with some values entered so you can see the large numbers I get. I had macros in the original sheet which I removed because they don't matter for this problem,so if something appear just keep macros disabled.

Don't know if I made myself clear but please let me know if you didn't understand.

Would be very thankful if someone can help me!