According to the chronology by Thiele, the kings ruled 211 years over Yisrael, including the 3-year rule of Avimelech, the first king of Shechem. If we add the 288 years of Moshiach ben Yosef, we reach 499 years, fathers and children together. That number is reached in 5789, without the Tikkun. This is already pretty awesome, but there is more.
On the other hand, the Tikkun of Moshiach ben David is, as we saw, 420 years, in 5786. Both David and Shlomo reigned for 40 years. (The other kings of Yehudah ruled over Tzion, but not over Yisrael.) Therefore, Moshiach ben David reaches 500 in 5786, fathers and children together. Moshiach ben Yosef dies, and Moshiach ben David takes over, without the Tikkun. That is awesome, indeed, but there is more: the Tikkun.
The life of the GR"A was to cancel the death of Moshiach ben Yosef, because what happens to Yosef, happens to Tzion, as has happened in the time of the fathers. His Tikkun makes Moshiach ben Yosef exactly equal to Moshiach ben David, in 5786. This is the meaning of the mention of the Even Shlemah, towards the end of chapter 3 of Kol HaTor. And it says there: ועי"ז ניתן הכח לרבנו להכשיר את האתחלתא בקיבוץ גלויות עד כדי להגיע לידי שלמות האתחלת. The Gematria of Moshiach ben Yosef and the Gematria of Moshiach ben David (דויד), together, is exactly 1000, and it is to be evenly divided. The tree of Yosef (E) and the tree of Yehudah {J} are combined, and made into one. "Behold, I am sending to you Eliyah, the Prophet", the GR"A. "He shall turn the heart of the fathers to the children, and the heart of the children to the fathers."
עוד יוסף חי, יוסף עוד חי, חי יוסף עוד, בן דוד חי וקיים
