NPS calculation formula

Our operational definition has NPS caluclated based on the total number of respondents. 

Stuart,
This is the Excel formula that I use to calculate the +/- error percentage for my NPS calculations.
IF(“NPS %”<0.5,(NORMSINV(1-(1-0.95)/2)*SQRT("NPS %"*(1-"NPS %")/"No. of Surveys")),(NORMSINV(1-(1-0.95)/2)*SQRT("NPS %"*(1-"NPS %")/"No. of Surveys")))
The sample size will then depend on the how tight you need the error to be. 
To give you an idea, if the NPS Score was at 50%, then a sample of 100 surveys would give +/- 10%.  A sample of 200 surveys would be +/- 7%.  It would take about 2000 surveys to get to +/- 2%.I  find that in order to get all the samples I need I have to use a rolling 6 month or rolling 12 month NPS Score.
 

What do you think? Is 50% plus 10% equal to 55%?

My question is to clarify if the 10% is 10 points of NPS (50 + 10 = 60) or is it 10% of the current NPS score (50 x 10% = 5…..50 + 5 = 55)?

“if the NPS Score was at 50%, then a sample of 100 surveys would give +/- 10%.”  comes from the post by Marty Yuzwa.  I am asking for clarification.

:)