The next correspondence to tempt us with his name was the Halloween Card mailed six months later and held the message, albeit part of the original card "I feel it in my bones, You ache to know my name, And so I'll clue you in." So likely having deliberately chosen this specific card, where is the clue.
The Zodiac Killer also adds BOO to the left of the skeleton, but does not write it in line with the original text, deciding to angle it in the same fashion as the BOO by the knothole. This, like the text in the tree lines up exactly with the letters ME, conveniently peering out beyond the skeleton arm. Was this a deliberate action on behalf of the Zodiac Killer, inserting his initials into the Halloween Card or more likely pure coincidence. Why did he emphasize the ME around the knothole or was it accidental.
More egotistical elements would be apparent in the Melvin Belli letter urging "please help me" on no less than three occasions and in his last confirmed correspondence described as the 'Exorcist Letter' mailed on January 29th 1974 the Zodiac would finally sign out, not with the Zodiac crosshair detailing his victim count, but the letters ME - 37. See here.
This is likely another case of searching for patterns where they do not exist, an affliction suffered by many who get embroiled in this case for any period of time, but there's no harm in trying. The answer may be an arms length away.