Just to make sure there are no misunderstandings: With any native, made-for-micro four-thirds, zoom, there is no need to input the focal length, as the camera and lens communicates electronically. So, when using micro four-thirds lenses, just leave that setting alone.
For lenses that doesn't communicate with the camera, it is best if you can input the exact focal length you are using. I know, that can be tricky or inconvenient you are using a zoom lens, but if the IBIS system thinks that you are using a longer focal length lens than you are, it will overcompensate and you end up with blurry pictures.
Personally, I would turn off the IBIS, and only turn it on when I wasn't using the zoom in my lens.