It would be pertinent to decide whether you plan to hang the blinds Singapore inside or outside the sill. It would be pertinent to mention here that you should hang the blinds inside provided the alcove on the window or door has been deep enough. For hanging inside the alcoves, you should take the height and width of the inside sill.
However, for hanging the blinds over the top and outside the alcove, it would be in your best interest to take specific measurements starting from the border of the door or window frame. It would be pertinent that the blind fits the space.