When I knew the Deirdre/Julia banner was coming up (i.e. the banner that just released), I actually made a spreadsheet of 100 trials to give me an idea if I should aim for one or both. I actually use the other summon simulator (this one: https://feh-stuff.github.io/summon-simulator/) since gamepress doesn't allow to create custom banners. Using the snipe function, you can actually generate data pretty quickly.
This summon simulator has some issues. Part of is is programming; Ethlyn is the normal pool for some reason, for example. But that's probably not a huge deal.
Gamepress also releases a 'should you summon' article that contains useful data. The most useful data point is probably the 90th percentile; roughly speaking, if you toss that many orbs that banner, there's a 90% chance you'll receive that unit.
In either case, 355 orbs were spent today to get Deirdre and Julia. I am literally only one Judgral unit shy--Ayra--of having them all. Surtr pitybroke me and the last time I had him I killed him for Steady Stance 4 so his summon quote was extra hilarious. I turned him into a book because I honestly just hate Surtr. Once I get steady stance 1 fodder, I'll think who wants Steady Stance 4. Thinking either Arden or Quan. Arden might prefer Steady Breath for that special acceleration but also the EP-guard really suits his playstyle; Quan has sturdy stance (+4 atk/def EP skill), so steady stance 4 might be nice for him too.