I am curious why you think Venom only has a slight chance of overtaking the Jumanji number? Just going on historical data, it seems most likely it will overtake Jumanji for the top film for Sony this year. All the films (besides Avatar and children's movies, as they would skew the numbers too much in favor of Venom) that had an opening weekend between 75 million and 85 million averaged a final box office of over 269 million. If you remove Passion of the Christ (as one could argue it was a bit of an outlier) they average over 259 million. If you take out the Harry Potters and Star Wars Episode II (since they may have a bigger multiplier due to its following) the average is still over 242 million. Only 3 movies in that range didn't end up with more than Jumanji (Solo: 84 million opening, 213 million total; Oz 79 million opening, 234 million total; DaVinci Code: 77 million opening; 217 million total).
I am not saying it is a definite, as we all know comic book movies are more front loaded than the average movie, but it certainly isn't unlikely. In fact, in support of what you said, of the 14 movies that had a bigger opening than Venom, 7 of those movies still had a bigger second weekend than Venom (on the other hand 7 of those movies Venom had a bigger second weekend then). It had the 4th smallest drop in it's second weekend of those 14 movies (Dr. Strange -49.5%, X2 -53.2%, Justice League -56.2%, Venom -56.4%). But as I said, comic movies are more front loaded than most movies. Of the 14 movies that made more than 80 million opening weekend and didn't beat Jumanji, 9 of them are comic book movies. I think it will be close, real close. This weekend will be the real sign of if it has a chance. The third weekend is where many of the comic book movies that didn't make it had it's biggest drop off (see Justice League).
Sorry, I love your post, and wasn't trying to be nitpicky. When you said that it just got me interested to look at the numbers. I haven't even seen Venom, and in all actuality I do think it will have trouble reaching that number due to a more limited audience compared to other movies in the same range as them.