The 'gap' starts at 20 years, but it actually widens as you go further back,
If I understand correctly, the gap between secular and WT dating is 20 years at 587 (Secular=587; WT=607)
At the deportation of the Northern Kingdom the gap slightly narrows (Secular=722; WT=740)
At the division of the kingdom after Solomon the gap is 67 years (Secular=930/1; WT=997)
At the exodus from Egypt (Secular=1446; WT=1513; Here the gap is holding at 67)
Secular dating actually helps explain certain things that WT dating leaves unexplained. For example, by secular dating Jonah visited Ninevah shortly after a famine and after an eclipse and during a very weak period in the Assyrian empire. These events may well have 'primed' the Ninevites to react favorably to Jonah. But WT dating puts Jonah in Ninevah way before these events.
By the way, I second the 'nice graphics' up above. Thanks for the efforts in those.