I believe the zoom intervals are a representation of your computer's operating system settings. I have no experience with Apple/MacOS, but I believe that in Windows, this is somewhere in system settings > Touch pad > Pinch to zoom(?) or in your 'Line numbers to scroll' using the wheel on a mouse. I can remember the exact settings as I'm not near a computer at the moment, but essentially it should be somewhere in your OS settings and not Miro. My number of lines to scroll was at 3 or 5 in Windows 10 and once I lowered it to 1, when I scrolled the wheel in the mouse while in Miro, it zommed in/out at lower increments.