To properly determine instrument error, you need to send your ASI to a shop that can calibrate it. You will get a table with calibrated airspeed versus indicated airpspeed (and an invoice!). The difference is instrument error. Only then can you say that your Van's airspeed (or one from another manufacturer), is off by XX knots. Once that is determined, you can proceed to find position, or static source error, using one of many good methods. Calibrated airspeed = indicated airspeed corrected for instrument error and position error.