I do 1-2-3-4-5, but it didn't work for me at first--it was uneven and unpredictable. The way I figured out what to do was as follows. I don't know if I can explain it in words, but I'll try.
I looked at the pattern of notes in the left hand, Eb-Gb-Ab-Db-Eb in the treble. The chord starts on Eb which is on the right side of a 2 black key block (Db and Eb), skips over 2 white keys to the next black key, Gb, then the next black key, Ab, then skips four half steps to the black key, Db, and finally the next black key, Eb.
Then I figure out how to mirror that pattern for my right hand. So mirroring Eb-Gb-Ab-Db-Eb going up, I play in descending notes--with my right hand--Db (half step from middle C), Bb, Ab, Eb, Db using 5-4-3-2-1. It's the same pattern the left hand has to play but reversed for the right hand (and on different notes obviously, it's the pattern that counts).
If I can play the mirror image of the pattern with my right hand, then it's just a process of figuring out what I'm doing on the right side of my body (hand, arm, shoulder, ...) that I'm not doing on my left. For me, I discovered a good deal of tension in the crook of my left elbow.
Then I start making up exercises for my left hand while it's crossed in front of my body, specifically in an effort to open up my left elbow. For instance, holding my pinky on the Eb, trying to play step wise with my thumb and fore finger several notes away. It turns out I also had stiffness on the inside of my upper arm. It's a slow process and sometimes intense, but it works for me.
Of course, we're all individual and none of this may help. I hope it does.
Otherwise, an alternate fingering.
