Hi Rachel,
Without specialized machinery, this certainly is a common hurdle with dried plant tinctures, especially any aerial parts as they are so light. There are a couple of things you can do here: 1. You can try blending your herbs into as close to a powder as possible before adding the alcohol. If you have already added the alcohol/menstruum then you may find that it still helps to blend the whole thing. Then follow by pressing the herbs down.
2.If this isn’t sufficient, you can start adding more alcohol/menstruum 1 part at a time until you feel good about the coverage and then note the ratio. So for your example, add 40 mL at a time until it covers the plant material and note the final ratio. I find I often end up with around 1:7
I hope this helps a bit.
Warmly,
Benna