There are two ways of doing this in Excel, depending on what you're most comfortable with.
The first thing you'll need is a CSV export of the NPS poll you ran. You can get this after the meeting is over by going to the Polls section, finding that poll, and saving the CSV. See the image below:
(If you're comfortable editing CSVs, you can also use the poll CSV export while the meeting is still running. Simply copy the results for the NPS poll into column C. Contact support in the lower right corner if you have any questions!)
Option 1: Single-cell calculation
The first option is to have the calculation done in a single cell.
Open the NPS votes CSV in Excel. Make sure the voting data is in column C.
Next, copy-paste the following equation into cell A1.
(This can actually go into any cell that's not in the C column.)
That cell will have your NPS score.
Option 1: Two page worksheet
To start, download the Excel workbook that will do the calculation.
You'll see some example data in the first sheet, and the actual calculation on the second sheet.
Next, open the NPS votes CSV you downloaded. Make sure that the format matches the example data (especially the column.)
Delete the example data. Copy-paste your NPS votes CSV where the example data was.
Switch to the NPS-calc tab. The Net Promoter Score is highlighted in yellow.