Originally Posted by Tenhou

It is random, no way to really know. Any SR counts though, so if you have recently won SRs from events you can try to time your collection of them together.

Yeah, that's what I do. Or I use my revival tickets since I know I'm going to be burning a CR. Or whenever they do a 2-3 day limited R-gacha 'event.' So in those cases, I know I'm going to be burning a bunch of R-gacha, but rather than burn them all on the same day, I make sure that I do it in pieces. Just enough on the first two days to get the Ex Wild. Then on the last day of the thing, I burn all of my R-gacha until I either run out or I get all of the limited cards.

During gambit, you can pretty much ensure the Ex Wild daily by opening gold/silver boxes for the SR. It might take a bunch of boxes, but you can usually ration your keys to get JUST enough to get it, then save boxes for the next day. Rationing is the key to most things.