From memory, seven 10-draws should get you one SSR on average. At the beginning of the year, they upped the chance to 3% for a few days and I was able to get 2 SSRs from that...
... Then after it was done, a Yuki gatcha happened and I was all out of jewels. Dang it, baseball season wasn't even close to starting. Going 2 for 40 was probably good luck though, even with double the odds.
Also interesting, the chance to pull exactly one SR from the first nine draws is the same as the chance to draw none at all.