A simple way to get fairly accurate numbers that I have used
You need to first and foremost calibrate your airspeed and get a good chart of CAS. Then you need to have a proper vertical speed indicator or sensitive altimeter with a proper static port location.
You can find the proper position by using tufts taped on your gyroplane's body and flying and noticing undisturbed areas on air where tufts seem to be straight back without any flailing around in coordinated flight. This is where a static port placement would be good. Having that all done, you fly at you suspected/expected best glide speed at safe altitude in dead calm smooth conditions without turbulence. Smooth wind is not turbulent so this doesn't mean no wind. It does not matter if you are flying in headwind or tailwind, just smooth wind without turbulence. At the suspected/expected best glide speed, turn your engine off (obviously be high enough over a landing spot like an airport n case engine won't start again). Then establish a constant and steady glide at that speed. Once steady, start a stop watch and simultaneously notice altitude. In exactly 60 seconds, note altitude again. That gives you your descent rate in Feet Per Minute (FPM).
Example:
Lets say you lost 1000 FPM. That is 11.36 MPH
Lets say this was at suspected/expected best glide speed of 50 MPH (that's 4400 feet per minute)
50/11.36 = 4.4
OR
4400/1000 = 4.4
You can repeat this in different headings one after the other 120 degrees or 90 degrees roughly apart and average the results
That's a simplified way. Technically you should use True Airspeed (TAS). A good approximation would be to add 2 MPH for each thousand feet of altitude to your Calibrated Airspeed (CAS).
A good way to calibrate your airspeed using GPS method that is an acceptable method by FAA is described in KitPlanes magazine article.
Are your airspeed indications accurate? By James B. Dabney.
www.kitplanes.com