To be honest, I'm not sure if there even is a breakpoint in Wheel of Fortune after which you are guaranteed the jackpot.

There are some events in the game that do have a set point after which you will get the rewards, for example, ninja treasures. It's supposedly just chance, but we now actually know you need about 700-1000 pulls to guarantee the super rare in GNW Treasure, and after that point it is a sure thing – no one's drawing like 3k pulls and not getting the SR. Last year during the big Christmas/New Year's/winter Fuku, the Lucky Snátch event had a specific breakpoint after spending 40k coupons (but only in certain conditions; it had to be all at once rather than over multiple sessions, etc.), tho that was apparently a bug.

In the case of Wheel of Fortune, I think it may be truly random. There have been cases of people getting the jackpot after spending 100 coupons, and cases where people spent like 50k+ and got nothing. Either we don't have enough data to determine the jackpot condition (which I find unlikely considering the amount of people playing, you would think we would have noticed a pattern), or it is genuinely random with no way to guarantee the jackpot payout.

I can tell you from anecdotal experience that I have gotten the jackpot in Wheel of Fortune once, but I spent somewhere between 30k and 60k coupons (it was during another big Fuku a while back so I can't remember specifically). However, if I were you, I would not take this statement as a confirmation that "if you spend 60k coupons in Wheel of Fortune you will get the jackpot." Honestly, I would not even gamble with trying to get the jackpot from it, and just keep saving. I'll also say from experience, that unless you have the ability to get the other 80-100 frags of the jackpot ninja, it is mostly useless – what good is 3* Obito Jinchuuriki doing me?