Looking for formula for survey percentage

[psdwit]

Morning Everyone I am hoping to get a little help. I'm stuck on my formula.
Using standard Net Promoter Score surveys I am looking to calculate how many promoters are needed to attain goal.
the formula we use is the following
P=Promoter
N=Neutral
D=Detractor

(P-D)/(P+N+D)
What I have so far is this
[TABLE="width: 500"]
<tbody>[TR]
[TD]A[/TD]
[TD]B[/TD]
[/TR]
[TR]
[TD]Promoters[/TD]
[TD]15[/TD]
[/TR]
[TR]
[TD]Neutral[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]Detractors[/TD]
[TD]2[/TD]
[/TR]
[TR]
[TD]GOAL[/TD]
[TD]85%[/TD]
[/TR]
[TR]
[TD]Current NPS[/TD]
[TD]=(B1-B3)/(B1+B2+B3)[/TD]
[/TR]
[TR]
[TD]# of Promoters Needed[/TD]
[TD]=IF(B5>=B4,"Goal Met")[/TD]
[/TR]
</tbody>[/TABLE]
I'm lost after the first part of the IF then statement in trying to calculate how many more of "B2" are needed if goal isn't met. I hope all this makes sense.
thank you for the help.

 

[hotabae]

The result will give me total of P needed to attain G not the additional number of P needed.​

Drsteele already gave you both of those.

Total of P needed to attain G:

B6=ROUNDUP((B2+B3+B3/B4)/(1/B4-1),0)

Additional Number of P needed:

B7=IF(B5>=B4,"Goal Met",B6-B1)

If you need them as one cell, just put this into your B6:

=IF(B5>=B4,"Goal Met",ROUNDUP((B2+B3+B3/B4)/(1/B4-1),0)-B1)