You could find out by using AbsolutePosition and AbsoluteSize
local function GetPositionRelativeToFrame(pos : UDim2, frame : Frame)
return UDim2.fromScale((pos.X.Offset-frame.AbsolutePosition.X)/frame.AbsoluteSize.X, (pos.Y.Offset-frame.AbsolutePosition.Y)/frame.AbsoluteSize.Y)
end
And it doesnt matter where its parented since it uses AbsolutePosition and size