It's very easy. Try this method.
You can pick up a destination easily from Last Destinations. Go to last destinations and select the destination you want to store into key, you will see a green rectangle on the selected destination. Next hold the desired programmable key for two seconds and all done.
With this setting, you can now easily go to the pre-selected destination on NAV from any screen, you need not be on NAV screen.
I set key #8 for Home, its the last key in the row and doesn't interfere with other keys and easy to remember as Final Destination is Home
this or that.