The Forecast for Leo(na) page mailed the following day was considered a play on the name and fate of "Leona", who wasn't murdered for at least a week after her abduction. At the end of December, the newspapers told of the discovery of her body at Bolinas Lagoon and refreshed the memories of the readers by reporting on the day of her abduction from the 749 Tormey Avenue apartment. Alongside the story of Leona Roberts in the Solano-Napa News Chronicle on December 29th 1969 was the headline "Zodiac Security Tightened", warning that the Bay Area murderer might strike again soon.
So it's probably unsurprising that this newspaper coverage was shortly followed by another "Zodiac" phone call on January 4th 1971 to Peggy Trainer, threatening her murder. Peggy Trainer lived at the same 749 Tormey Avenue address from where Leona Roberts was abducted from. The Zodiac Killer could easily have read the newspaper coverage and phoned a death threat to Peggy Trainer to attach himself to that address and the murder of Leona Roberts, but if this phone call was all he did, then how lucky was he that his pseudonym "Zodiac" was mailed by somebody else on December 10th 1969 (the day of her abduction), who pasted "Birds Fly South" which appeared to coincide with the north-south Pacific flyway of Bolinas Lagoon where her body was found, and who mailed a "Forecast for Leo" page that coincided with her name and not immediate death.
The notion of an abductor, forcibly removing a 16-year-old girl from a residence and securing her at an undetermined location, manufacturing an immediate multi-pasted communication, and then driving 58 miles to Sacramento to mail the letter, for it to somehow receive a December 10th postmark, is totally unrealistic. The fact that the designer of the two December communications chose horoscope pages from November rather than December - which would have been more relevant to the crime - could suggest that these two letters were prepared in advance of Leona Roberts abduction and the generic Cancer page mailed prior to 6:15pm on December 10th 1969.
After the successful abduction had been achieved, the more specific "Forecast for Leo" page could then have been dispatched. The other alternative, is that neither communication had anything to do with the abduction of Leona Roberts, and the phone call on January 4th 1971 was unrelated, but for the pseudonym "Zodiac" that was relevant to both. It has yet to be definitively established whether any connection exists between the abduction of Leona Roberts and the December 10th/11th letters, or any Zodiac involvement whatsoever. The communications and phone call on January 4th 1969 could have been somebody attempting to misdirect law enforcement away from a killer known to Leona Roberts, who had personal reasons to end her life. Implicating the Zodiac Killer for a murder (especially by phone) was an easy option through 1969 and beyond. I think it much more likely that the murderer of Leona Roberts was a mentally inadequate individual with an unhealthy fantasy he failed to control. Sounds just like Zodiac doesn't it.