According to reports, the +100% eidolons have a lower draw rate than other SSR eidolons. Since my Gatcha calculator assumes that the draw rate of each SSR eidolon is the same, it won't correctly predict your odds of drawing a +100% eidolon.
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I got my info from a DMM player. I'm not sure if it's been documented, at least in the JP wiki. Assuming they can be obtained from Jewel Gatcha, we should find out the draw rate from rerollers soon enough. I suspect we'll have two options to get one: spend thousands of dollars on Gatcha, or reroll. The draw rate should be too low for players to get them through normal play, or even by paying a reasonable "subscription" of $30/month.
Do you know if this also holds true for the SSR Eidolon tickets? If that is not the case i'd imagine you would eventually get your choice. If all are equal the odds of landing any single one would be 1/27... when all 100% eidolons are released. Assuming price remains 3,000 gold a ticket it's not incredibly terrible, but still spendy, that said, probably still better than when they are featured because you still would need to roll an SSR, then have it be an eidolon.