This guy's description may be more clear:

Model 3 Range Constant Check
But check my figures below as well.

Almost. Other than these I need to know how much was the range decrease on the battery monitor. I made a quick guide.

View attachment 471016
Notice that the longer you drive the more accurate the result is.

If you only use up 20 miles rated range, then the counter could stay at 20.4 or 19.6, that could generate +-2.5% error which is too much. This is why I said, if you can't make a long trip, reset the trip meter when the range meter drops this way you know it's let's say 260 instead or 260.4 or 259.5. And similarly end the trip at the time when the range meter drops.

If you can make a long trip like 250 miles, this won't matter much as the maximum error would be 1/250 = +-0.2%

The average consumption on the trip meter is a rounded number as well. That might give another +-0.4% error.

Keep the consumption at around 250Wh/mile during the test to have an accurate measurement. Turn off AC or heating if possible.

The usable battery capacity on a brand new Model 3 LR (RWD/AWD/P) is 74.5kWh.

In the example above I used Tesla Bjorn's numbers, his car has around 6% degradation (even though the rated range is at 484km = 300.6 miles) at 25k miles.

If your number is way off, then you either didn't follow the procedure or some metering system is wrong in the BMS.

Also your Model X has some degradation too, but the range meter doesn't show it. It's impossible not to have some. If I recall, the battery capacity in that car is around 98kWh when brand new.