Unless you use web publishing, you have to use FMGo12 if you have converted your database to fm 12.
You can use the following script to set a global variable and/or go to the correct opening layout for a given device. If you set a global variable, other scripts that change layouts can refer to the global variable to determine the correct layout.
Set Variable [$Version ; value: Get ( applicationVersion ) ]
If [PatternCount ( $Version ; "Pro" ) or PatternCount ; "Runtime" ]
Set Variable [$LayoutSet ; value: "Pro" ]
Go to layout [First layout for FileMaker Pro/runtime users here]
Else IF [ PatternCount ( $Version ; "Ipad" ]
Set Variable [$LayoutSet ; value: "Ipad"]
Go to layout [First layout for Ipad users here]
Else If [PatternCount ( $Version ; "Go")]
Set Variable [$LayouSet ; value: "Iphone"]
Go to layout [First layout for Iphone users here]
Show Custom dialog ["OOPs, can't figure out what platform opened the database."]