Other than to check for blocked arteries, I have never heard of anyone using a blood pressure cuff anywhere except their arm. (You have to keep in mind that the ankle is lower and has more gravitational pressure so the blood pressure should be much higher there.)
The Ankle Brachial Pressure Index (ABPI) is the ratio of the blood pressure in the lower legs to the blood pressure in the arms. Compared to the arm, lower blood pressure in the leg is a symptom of blocked arteries (peripheral vascular disease).
If I were you, I would stick to taking reading from the arm.

Well i had about the same issue...bp at arm 110/73 and at ankle 140/90 till i lay down to let the arm and ankle be at the same height. in this posture (lying down) the ankle bp was 117 and arm 107 mmhg.