I never verified it, but I was told that:
7 point dogs win 1time vs 2times (1/3rd, 33 of 100)
14 pt dogs win 1 vs 4 (1/5th, 20 of 100)
21 pt dogs win 1 vs 8 (1/9th, 11 of 100)
23 out of 100 is even better than the 20 out of 100 of the rule of thumb I was taught.
But for perspective, #5 Miami...