Why 76%?

5 is in the mind family (1, 5, 7) and 3 is in the heart family (3, 6, 9). Mind with heart starts at 74. The nudge comes from the roots: 5 + 3 = 8, which leaves 2 when divided by 3, so the nudge is +2. 74 + 2 = 76.

Out of the 78 possible pairings, 21 score higher than this one. Rarity: 11.1% of birthdays from 1950 to 2009 give 5 and 11.1% give 3, so about 1 in 40 random pairs of birthdays land on exactly this pair.

The rule is our own design, not a numerologist's chart, and numerology is not a science.