Commonly used arm positions can overestimate blood pressure readings: study

Here's how:

Lie down on a bed on your back, and put the cuff on your arm and get the 'button' within reach of your finger to turn it on. Then completely relax (and DO NOT move) with soft music or whatever. Then without moving your body at all, after 10 minutes (at least) push the button to start the pressurization and reading.

This gives an accurate reading and is often DRAMATICALLY lower than if you don't do it this way. I was convinced I had super high potentially life-threatening BP until I learned this.

Now if you want to see I'm right, get up and walk around some and then sit back down and take another reading. It will be noticeably higher, because your heart starts pumping harder even from minimal movement.

Just because it gives lower results doesn't mean it's more accurate. I can raise my arm during measurement or use a tourniquet above the cuff and get a lower result. That doesn't mean the measurement is accurate at all.

There are standardized procedures on how to measure BP. Your "accurate" method is not one of them.