take enough to cover the bill PLUS the change for your tip. 
I went out to eat with a friend to Friday's and my friend paid her portion (about $
on her credit card and I stuck a $20 in. Waitress came over and I said, "Can you put $8 on the card and use the cash to cover the rest of the bill?" She comes back and put $8 on the card and took all of the $20. My portion came to like $16 so I said whatever, it was all over a few bucks. My friend just gave me what she was gonna leave for tip and we left her nothing more since she got way over 20% with taking my change.

I went out to eat with a friend to Friday's and my friend paid her portion (about $
on her credit card and I stuck a $20 in. Waitress came over and I said, "Can you put $8 on the card and use the cash to cover the rest of the bill?" She comes back and put $8 on the card and took all of the $20. My portion came to like $16 so I said whatever, it was all over a few bucks. My friend just gave me what she was gonna leave for tip and we left her nothing more since she got way over 20% with taking my change. 