The BP’s I took were always different in each arm. Regardless of whether sitting or standing, the only ‘constant’ was that when chest discomfort started, my diastolic BP reading was always above 90. The pain and chest tightness I would get would be on a latitude of just above half-way up the breastbone, slightly to my left.
Hope you are managing to eat at least one banana a day, as you definitely need as much dietary potassium as you can get, looking at your reading. Another source of potassium is fresh orange, or fresh orange juice.