The sweet spot for my scosche is my upper arm just above my elbow with the sensor right in the middle which is where the brachial artery is

Took a while to get the right tightness on the strap but it’s perfect pretty quickly

I had much more trouble w forearm both low and high

I just bought a 945 so I won’t be going the 255 route

My 935 is average at best w running-it does stop tracking step rate after a good warm up usually