[Pyjam]

I've struggled a lot to obtain what I want. Finally, the best and easy way to get what you want is to edit the AZW3 file. Don't be affraid. It's very easy.

1. Edit the book, format AZW3
2. In the list of files on the left, you'll see a .css file, double clic to open it.
3. You'll see a list of styles with their attributes. Usually, the main style is:

.calibre {
...
font-size: 1em;
line-height: 1.20;
...
}

4. Adjust the font-size to what you want. If you put 1.10em, for example, all characters will be 10% bigger.

5. change line-height here and also everywhere in the file. This parameter fixes the minimal spacing. So if some part of the text uses multiple style (it's often the case, the biggest value will be use).

6. Then quit and save.

When reading the book with your Kindle, select the tightest spacing.

for example, I have:
font-size: 1.065em;
line-height: 1.45;

That way, I have 22 lines of text with the font Bookerly and the simple spacing between lines. Perfect for me.