Is this a test used by doctors to help point to a possible diagnosis of tos? My blood pressure markedly falls with my arm raised above my head. Both values, but the diastolic slightly moreso.

It is not the only test or even a definitive test... about 25% of normal population will have some blood flow limitation with arms raised.
It is a indicator to follow up with , especially if concerning symptoms are present.

My arm blood flow cuts off with arms raised still, but I have fewer & mild symptoms now.

Unrelated, I'm a biomed student and I've heard about something called Mayne's Sign; a now defunct/useless part of a physical examination looking for signs of aortic insufficiency. This sign is positive when there is a drop of diastolic blood pressure greater than or equal to 15mmHg with arm raised above head. Apparently thoughm, this is seen in 65% of normal individuals hence why its been thrown at as a clue to aortic valve problems.

Anyhoo, like I said I'm seeing both values drop, but the diastolic drops slightly more. Its certainly in the 15-20mmHg range as well! Heh. I have been referred to a hospital outpatients clinic by my GP so I will mention this to the doctor I see there. My main problem is my right side and the headaches that have been almost daily since the middle of December but I do get some mild cooling of the right hand and slight discoloration.