There is no definitive answer to this question as it depends on a number of factors, including the type and size of your boat, the type of fish finder you have, and your personal preferences. However, here are some general guidelines to help you decide where to mount your fish finder screen.
If you have a small boat, it may be best to mount the screen near the helm so that you can easily see it while steering. If you have a larger boat, you may want to consider mounting the screen in a more central location so that everyone on board can see it.
When deciding where to mount your fish finder screen, also consider how easy it will be to reach and operate the controls. You don't want to have to take your eyes off the water for too long in order to adjust settings or view information.
Finally, think about what kind of fishing you'll be doing most often. If you're primarily trolling for fish, then mounting the screen near the stern might be ideal so that you can keep an eye on it while baiting hooks and setting lines. If bottom fishing is more your style, then mounting the unit near the bow might give you a better view of what's going on beneath your boat.
No matter where you ultimately decide to mount your fish finder screen, just make sure that it's in a location that's convenient for you and provides good visibility.