Who's the best in that time frame has always been debatable. According to your stats, Austin looks the best. However, it all depends on how much weight the Slams weigh. Obviously, the '79 and '80 French had weak fields and only the '81 Australian had all the top players. So, if you take only Wimbledon and the USO from '79 and '80 plus all four from '81, look at this.
Evert 6 of 8 finals, 2 SF; 2 wins (plus 2 watered down French titles)
Navratilova 3 of 8 finals, 3 SF, 2 wins
Austin 2 of 8 finals, 3 SF, 2 wins
That's probably where Evert made up some ground on the other categories. Obviously, Austin had Evert's number for awhile. I'm an Evert fan but not so close-minded to think that there's a case for Austin here. Not Martina though. She'd have her time at the top.