Scale to match the camera? Are you just trying to move dummy to camera target position or something more?
Something like this would work in a single player map (requires vJASS):
library CameraDummy initializer init
globals
private constant integer DUMMY_ID = 'hfoo'
private constant player DUMMY_OWNER = Player(0)
private unit dummy
endglobals
private function move takes nothing retuns nothing
call SetUnitX(dummy, GetCameraTargetPositionX())
call SetUnitY(dummy, GetCameraTargetPositionY())
endfunction
private function init takes nothing returns nothing
set dummy = CreateUnit(DUMMY_OWNER, DUMMY_ID, 0.00, 0.00, 0.00)
call UnitAddAbility(dummy, 'Amrf')
call UnitRemoveAbility(dummy, 'Amrf')
call TimerStart(CreateTimer(), 0.03125, true, function move)
endfunction
endlibrary