Finding the right location for your Normal Button is a complex subject, with many known and unknown variables. Accredited Installers are required to take part in years of training, and even then in some cases it takes years of experience on the job to fully appreciate the many factors involved. As in all cases, safety issues are paramount.
Many people choose to install it in the vicinity of other utility and safety devices, such as consumer units (fuse boxes), fire alarms or utility cupboards. This is often an excellent choice, but be aware of the risk of electrical, spatial and conceptual interference. Learn to ask yourself the right questions. Could the unit be misused by a stranger or accessed by a young child? Is there a water inlet nearby? How will you manage standing waves?
A successful placement will have the right balance of access, security and hazard mitigation. For every general rule there are exceptions. We've even seen successful installs on office desks and kitchen walls.
Ultimately, as with any other home or workplace installation, if you're not feeling confident it might be best to hire a professional.