Using the NormSDist Function to generate an S Curve

[singcbl]

I been trying to find a simple way to generate an S curve and came across the NormSDist function but I find it daunting. I have searched on the web in other forum but somehow could not find the answer I need.
I have a project which has a certain value let says $1,000,000 which is projected to complete within a time frame of between 12, 14 or 16 months. So what I need is to populate the monthly amount need to be completed within the three different time frame. From past experience the amount completed each month follow very much like the "Bell" curve. So if I can generate a bell curve over the 3 different time frame I should be able to create a S curve by accumulating the monthly amount.
I be grateful if anyone of you can show and guide me how to do this.

Thank you.

 

[joeu2004]

Thanks to Joeu2014 suggestion I have a workable solution but I would to know if this solution can be further simplify in that instead of having to 2 formulas in this suggest I would very much like to have just a formula for the monthly cashflow amount. The accumulative cashflow can be easily generated. Thank you once again for all your help.

The formula in C8 is =IF(A8="","",B8-N(B7)). You can simply replace the reference to B8 with the formula therein. To wit, in C8:

=IF(A8="", "", $B$1*(NORMDIST(A8,$B$3,$B$4,1)-
 NORMSDIST(-$B$5))/(NORMSDIST($B$5)-NORMSDIST(-$B$5)) - N(B7))

 

[singcbl]

Thanks for the help.
I wonder if this can be converted into a function macro. I think it will be make it easier to understand.

 

[singcbl]

Joeu2004,
Oops! When I did what you suggested and copy downwards the C8 to C9 and beyond the answer don't seem to be correct.

 

[joeu2004]

When I did what you suggested and copy downwards the C8 to C9 and beyond the answer don't seem to be correct.

Yes, my bad! Sloppy! It is not a simple fix; and perhaps I can do better, if I start from scratch. I'm busy most of today. I'll get back to this tonight.

 

[joeu2004]

Yes, my bad!

In the original example Excel file (see comment #8 in this thread):

C8: =IF(A8="","",$B$1*(NORMDIST(A8,$B$3,$B$4,1)-NORMSDIST(-$B$5))/(NORMSDIST($B$5)-NORMSDIST(-$B$5)))
C9: =IF(A9="","",$B$1*(NORMDIST(A9,$B$3,$B$4,1)-NORMSDIST(-$B$5))/(NORMSDIST($B$5)-NORMSDIST(-$B$5))-SUM($C$8:C8))
Copy C9 into C10:C23