What scale is the speed trend line on a PFD's ASI?
I found some training material on Airbus PFDs which explains:


  A vertical arrow, called the speed trend arrow, indicates the aircraft's
  predicted speed in 10 seconds if the current acceleration or deceleration rate remains constant.


Put another way, if you measure the acceleration in knots per second, and then multiply it by ten, it's on the same scale as the speed tape.

If you want to reach a particular speed in ten seconds' time, adjust pitch or power to put the head of the arrow on that tick on the speed tape, then keep adjusting to keep it on the same tick as it approaches.