Cones measure the combined effect of temperature and time, (heatwork). They are sometimes referred to as witness cones.
To make a more accurate assessment of your kiln’s performance use three cones as illustrated. The target cone is 06 in the middle and is down perfectly; the guide cone, number 07 is the one on the right, it is one cone cooler and is over fired; the guard cone number 05½, is the one on the left, it is one cone hotter and is underfired.