I take a low dose (7.5mg) of mirtazapine off label for insomnia, which I have used for over 10 years.
If I were you I would start with 3.75mg ie a quarter of a 15mg tablet, at night if you are sensitive to medication.
It doesn't do much of anything at this dose although some do find this effective, and why take more than you need of any medicine if you find this adequate.
This will help assess your tolerance before moving up in dose, which I would do gradually.
I would say 7.5mg to 15mg is maximally effective for insomnia generally, above this is considered to be more activating although I have no personal experience of using more than 15mg.
In my opinion it is still better to start low, at night, than risk a 30mg dose off the bat if you tend to react negatively to medication because that could be far more unpleasant if you don't get on with it, even if it doesn't result in sedation.