Given your intended use of the data, may I suggest an alternative metric instead of L/D?
Is your aircraft a quad plane?
Also is the forward propulsion electric?
On the assumption that your aircraft is electric and you have current and voltage sensors attached and suitably calibrated, then a suggestion would be for you to use power and thrust as your metrics as these directly correlate to max endurance and max range flight conditions.
It is known that the maximum endurance of a fixed wing plane coinsides with the minimum power required for straight level unaccelerated flight. In my experience the velocity at which the aircraft will need to fly at to achieve this condition will always be quite close to the stall speed of the aircraft (within 20% of the stall speed). You need to make sure that your aircraft has a pitot tube for measuring airspeed and that you have adequately tuned ARSPD_FBW_MIN to be close/equal to your stall speed. Then you can run a series of experiments in which you leave your aircraft in loiter (with a wide turn radius) and fly it at series of discrete speeds. Then using the logged data you can correlate the velocity that flys at min power required.
Similarly to the above, the velocity that achieves maximum range in a fixed wing aircraft travelling in straight level and unaccelerated flight will coincide with the minimum thrust required. For a propeller aircraft it is a reasonable approximation to say:
thrust = power / velocity
Therfore using the same data from the flights suggested above you can now do a bit of work with the log flies to determine the velocity that corresponds to max endurance. And if you really want to find a lift over drag value it just so happens that to achieve max range you would need to fly at L/D_max and you could determine it from this equation:
L/D = weight / thrust
This is of course only applicable if flying at constant altitude and constant speed.
Using this method has the advantage that you are specifically flying in cruise conditions (your region of interest) and it reduces the effect of wind speed and direction, which would have a big impact on glide slope based approaches. Also you can fly your aircraft in loiter for longer periods of time, collecting more data, to improve your averages. Thus the experiment will likely have greater repeatability.
The disadvantage of this approach is that it relies on power and therefore current measurement, which is not the most reliable and will typically give noisy information. However, given that the experiment requires your aircraft to fly straight level and unaccelerated the current draw will be pretty constant and the measurement will be quite reasonable. Additionally, if you take the mean of your data for each flight velocity and you find that when you plot out velocity vs power (or velocity vs thrust) for all experimental conditions and you see a minimum power/thrust that occurs somewhere close to stall speed then it is a strong indication that the experiment worked and that the method’s experimental noise is reasonable.
If you do have a quad plane, use Q_assist and be sure to set Q_ASSIST_SPEED to be your stall speed. Then the plane will always be able to get out of trouble if it does stall.
I have messed around with getting my own quad plane to fly at slow speeds and was impressed at how well FBWB based modes fly at high alpha when an air speed indicator is fitted.
I hope all of that made sense. Not sure I explained it very well