1. Win all of the Lean Republican and Likely Republican seats, thus picking up 25 Democrat seats which are currently in those categories,
2. Win 2/3 of the Democrat toss-up seats, thus picking up 32 more seats, and
3. Win 20% of the Lean Democrat and Likely Democrat seats, picking up another 10 seats.
This gives us a total of 25 + 32 + 10 = 67 seats.
Assuming the Democrats pick up the 3 Republican seats rated Lean or Likely Democrat, that gives the GOP a net pickup of 67 – 3 = 64 seats.
This would be the largest gain for either party since 1948.