On letterbox mode have an event on start of layout scroll to x, y then set those values depending on your layout size.
For example if your layout size is 640x480 then that event would be "on start of layout | scroll to x:320 y:240
You have to put that event in the event sheets of all the layouts you have, and if all your layouts are the same size then you can just create a seperate event sheet(no layout needed) put the code there and then include that event sheet in all your other event sheet.