Usually fuku deals that are past 20k will have ninjas close to that amount in events which means they will make the limit much higher for all of the 80 frags. Ninjas in 20k fuku deals tend to cost less than 15k in events.

So you can't predict the next limit for when fuku deal comes again. You can only predict how far has the limit has gone and you can decide to save minimum 20k or go for the full 30k.