If sensing the ajna chakra is your aim, you may find this method useful too. It is a combination of Jallundar bandh (chin lock) and trataka (concentrated gaze).
Sit with a straight back, eyes closed lightly and let your head go down slowly. (breathing should only be slow and easy; but no holding or deliberate deep breath) As chin touches the chest, start tilting your gaze upwards while keeping the head firmly on the chest. The gaze must be tilted as if you are “seeing” something; just a wee bit of stretch, no stress. Automatically your gaze zeroes in on a spot about half inch inside and half inch above a point where eyebrows could intersect. That’s ajna.
Hope, you are also aware that chakras only have approximate co-ordinates on the physical plane and they move their location at different times.