My mum is on 15mg. I think some can tolerate 30mg with the same level of 'urge control' but higher than that and it doesn't do the trick.
I found quite a good research paper on it not long ago where it was tested on pwd groups.
I give my mother her Mirtazapine about half an hour before bedtime. It's great because she's not 'hungover' the next day. I've got a small supply of sleeping pills to use on an 'as needed' basis in emergencies but I haven't needed them so far.
The consultant did emphasise that it doesn't always work for everyone. If it failed for us he was willing to gently trial other things.
But I have to say he's an awesome consultant with a very profound understanding of the nature of dementia. I feel very lucky because as some of you know, I was on the floor with exhaustion.
I hope you find a workable solution. No sleep totally kills me.