I usually have to go a fair bit finer (8 on my niche isn’t unusual), but have had good results from decaf beans. I’d keep going finer until you hit the ratio you want, or choke the machine and then work back.
I assume the decaf beans are freshly roasted? Old stale beans often run very fast and are hard to dial in. If you’re buying beans without a roasted on date (I try to use all beans within 6 weeks from roast date, or freeze if that isn’t possible) then perhaps turnover is lower on the decaf and they’re older than what you’re using for caffeinated.