the Air Speed Indicator is a primary instrument that should be used as the 'stick position indicator'. Staring outside at the horizon slicing the wind shield is the wrong technique in relation to air-speed. Sure it works fine over a dead flat horizon, but it is still the wrong technique. Try that in serious Mountainous terrain & I guarantee You that You WILL loose vital airspeed & likely to die unless You have huge reserves of power.
Ask Yourself what speed do I want??? Then adjust the stick position accordingly; based on the ASI information (only)
All other Visual Flying information should be based only on what You see outside
after 35yrs flying 10yrs in extreme Mountain terrain You can be rest assured that this the only safe technique for airspeed control!!!