and move the window 112 and the indicia 114 the window 112 and the indicia 114 are in fixed relation to one another the sprinkler drop ruler 100 may be calibrated by moving the window 112 so that the indicia 114 fixed relative to the window 112 moves as well when the top 106 is flush with the rigid frame 102 the window 112 is moved until the indicia 114 point to u201c 0 u201d in the ruler fig5 illustrates one example of adjusting the window 112 in which the window 112 may be moved either up or down based on one or more guides or rails 120 in practice the window 112 may be moved so that the indicia 114 points to u201c 0 u201d when the top 106 is flush with the rigid frame 102 another way is to move the extension piece 110 along with the window 112 and the indicia 114 the extension piece 110 the window 112 and the indicia 114 are in fixed relation to one another the sprinkler drop ruler 100 may be calibrated by moving the extension piece 110 so that the window 112 and the indicia 114 which are both fixed relative to the extension piece 110 moves as well in practice when the top 106 of the ruler 116 is flush with the rigid frame 102 the extension piece 110 may be moved upward or downward.