In today's instance of 'maybe I should've looked this up earlier':
So I've been hoarding up SR eidolon spirits in preparation for the next couple of raid eidolons. Particularly, I stockpiled a lot for Amaru, thanks to Gluttony. And by a lot, I mean 120 SR thunder spirits.
And only just now did the thought occur to me to check the experience table page to see exactly how many I would need. Turns out it's 102 matching SR spirits to get an SSR from 1 to 100 without rolling any super successes. Well, that frees up 18 slots then. Not a big deal, but it'll help the frequency of having to burn N trash when doing the gem gacha drawing.