Here is an example of why 20% for servers is necessary:

When a server rings $500 for the night (which was average) 7-hour shift
Then the server will (tip out) pay about 4% for busser, host and bartender = $20.00
The I.R.S. will assume the earning 12.5% of the sales and tax $62.50
And if the server worked 7 hours 7 x 2.13hr = $14.91
Income tax @18% for 62.50+14.91 = $13.93
Total tip out by restaurant and tax taken out by I.R.S. = $33.93

If the server received average of 15% as tip for the night = $75.00
then the server will walk home with $41.07

If the server received average of 20% as tip for the night = $100.00
then the server will walk home with $66.07

When a server rings $1000 for the night 7-hour shift
Then the server will (tip out) pay about 4% for busser, host and
bartender = $40.00
The I.R.S. will assume the earning 12.5% of the sales and tax $125.00
And if the server worked 7 hours 7 x 2.13hr = $14.91
Income tax @18% for 125.00+14.91 = $25.18

Total tip out by restaurant and tax taken out by I.R.S. = $65.18

If the server received average of 15% as tip for the night = $150.00 
then the server will  walk home with $84.82

If the server received average of 20% as tip for the night = $200.00
 then the server will walk home with $134.82