The ranking system in 1989 was not that much different than the one today. Lendl won 10 tournaments, including the Australian Open and Key Biscayne, Hamburg, and Montreal (which are all Masters Series events today). While Becker won Wimbledon and the US Open, he did not accumulate enough points to rightfully be #1... and he wouldn't be #1 even in the system as it stands today. (In fact, Becker didn't actually get the #1 ranking until two years later... in early 1991.)
I looked through my old tennis magazines recently & found the points differential for 1989. The ATP ranking didn't count any WCT events or the Year End Masters. It was a system that averaged the points you earned at all other events by the amount of events you entered.
The final standings:
Lendl 213.214 from 15 events
Becker 189.916 from 12 events
Edberg 150.857 from 15 events
Using the points listed on the player activity for '89 for these players on the website, the average I came up with matched the average listed in Tennis Magazine.
Also Tennis Magazine published the totals of the players under the Grand Prix Race system for 1989(this includes WCT & the Year End Masters) The top finishers in this system qualifed for the Year end masters:
Lendl 9,831
Becker 7,039
Edberg 6,355
Under both ranking systems, Lendl had a big lead over Becker. But Edberg gets closer with the 'race' system(not surprising since he won the Masters that year) I'm curious as to how different the atp ranking for that year would be if the Year End Masters was included(in which Becker made the final)
And who knows, Wilander may have not passed Lendl in '88 if it had, since Lendl was so dominant at that event, an event which gives so many points today.
Also looking at the player activity page, you can see how many points were given to the majors back then. I'm assuming bonus points were used since they all gave different amounts to the winners of each, though there does seem to be a big gap in points with the AO compared to other majors.
Becker got 466 points for winning the USO in '89
He got 488 for winning Wimbledon.
Lendl got 335 points for winning the AO.
Chang got 455 for winning the French.
Here is Becker's page for '89:
http://www.atptennis.com/5/en/playe...1989&player=B028&selTournament=0&prevtrnnum=0