The flight procedure I use is the 2005 USAR handicap procedure image above. Once at wide open throttle, max RPM, leaned for max power and trimmed for hands off level flight at 6000 ft density altitude for EVERY TEST OF SPEED
I engage the autopilot heading and altitude hold and start recording GPS ground speed for the 360 deg. GPS ground track until I have 5 consecutive 20 second interval recordings that do not vary by more than one GPS ground speed knot then I turn 120 degrees and repeat for the next leg until I have completed 3 legs then I return to base with my data and plug the valid leg average GPS ground speed for the five recordings for each leg and the GPS ground track for each leg into the National Test Pilot School (NTPS) Excel spreadsheet that was provided in this forum. Embedded formulas compute the wind experienced during the test and the true air speed of the airplane.
Originally I used the handicap procedure alone bur Kevin Horton convinced me that that procedure would always error on the side of lower speed. I verified that in actual use and have used it with the NTPS excel file ever since. John Huft also supported the use of the NTPS spreadsheet and recommended multiple runs before accepting a result as accurate. I verified again that the suggestion was correct for refining the estimate but there are practical time and money limits to my resources so I now fly two triangles, one right after the other, and average the two estimates. The estimates are always very close but this approach dampens out the variations.
I now fly my tests under a MOA east of the Fayetteville, Arkansas's Drake Field and 6,000 ft density altitude is often around 4,000 ft MSL and ~2000 ft AGL over the Ozarks there. Wind and thermals during tests in non calm conditions occasionally cause a speed excursion outside the acceptable range of the USARHP and I compromise my standard by recording the observation and continue with data collection until I have a five good consistent recordings around the one anomaly and strike it from the list when I get back on the ground.
A few more more important lessons learned:
Try to minimise variations in the aircraft under test that are not the item of interest in the test (Fuel load, aerodynamic fairing installations, tape, vent position, etc.).
Always record the date of the test on the raw data collection sheet.
Always record what is being tested on the raw data collection sheet
Always print the NTPS spreadsheet for the test and staple it to the raw data sheet for the test.
Always File the paperwork.
I hope that helps.