I use .357”
Worst thing you can do with a cast bullet is undersize it, besides not lubricating them.
As long as your rounds plunk and feed, bigger is better.
If using scrap or w/w, get some 95/5% (tin/antimony) lead-free solder to harden and improve mold fill.
I used .356 for nearly 30yrs, but decided to try .357”. Accuracy improvement surprised me.
And, btw, it’s not 0.9mm, it’s just 9mm.