Beast Mode for Top 20 then Other

I am creating a table to rank sales by each company for the last 3 years and totalling the revenue. 

I would like to show in a table the Top 20 companies by Total 3yr Revenue with the remaining companies aggregated in a row named "Other" rather than the individual company names.

I was trying a beast mode to rank each company by their individual total revenue divided by the total revenue for the entire dataset for a percentage, but I can't seem to get it to work.

Please help! I know how to do this with properties in a pie chart, but not in a table. 

Thank you in advance!

Example table:

Company2015 Rev2016 Rev2017 RevTotal Rev
Co A$1,000.00$1,000.00$1,000.00$3,000.00
Co B$900.00$900.00$900.00$2,700.00
Co C$800.00$800.00$800.00$2,400.00
Co D$700.00$700.00$700.00$2,100.00
Co E$600.00$600.00$600.00$1,800.00

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