How is the minimum score for Rank Rewards calculated for Summoner Advancement Event?

There are different tier list in summoner advancement event (and not only this event). The way it is defined is not clearly explained by Kabam, but we can assume that it is based on the prestige of the alliance.

What is the prestige ?

It is the sum (or maybe the average of the sum) of the prestige of the member of the alliance, prestige of a member is based on the PI of his most powerful heroes.

Why is this system made for ?

This way, low/small/new alliances can also get some rewards from summoner advancement without being overwhelmed by big alliances. According to the way points are calculated in this event, in a single ranking system, it won't be possible to a small alliance to gain rewards from this event (opening big crystals (that will make you earns many points) every week is not possible until you have a big level)...and it wouldn't be fair.