Mouse Scrolling for Zoom

Hello everyone!

I'm using a Macbook Pro with a Windows mouse and C1 Version 20.
My problem: After using the scroll wheel in the viewer window to have a closer look on the detail of my photo it's zooming to 400% instead of the 25% that I'd very much prefer. There are no steps in between and I cant find a way to change the intensity or the steps of the zoom process.

I am looking forward to hear your thoughts and suggestions on this! Thanks very much :)