Average BG meter error
Every BG meter has some noise in the signal it measures. In fact, what constitutes the system under measurement is [your finger + your BG meter + you], and results may vary for the same BG meter depending upon the person using it.
Here is a procedure you can use to figure out how to estimate the error you should expect to see for [your finger + your BG meter + you]

When you are stable around 70, take 15 consecutive BG measurements, using different fingers, trying to make a fingerpoke every 30 seconds (as fast as is reasonably possible, really, so you donâ€™t drift too much).

Plot them very carefully, using a very large scale, on graph paper

Draw a straight line across them that averages out the signal (it may not be quite horizontal)

For each measurement you took, measure the difference (the â€śerrorâ€ť) between the measurement and the line

take the ABSOLUTE VALUE (i.e. whether it is positive or negative, use the positive value for each measurement) of each error

add all 15 up

divide the result by 15.

The final result is your average error around 70

Do the same thing when you are stable around 120. That will give you the average error around 120.

Do the same when you are stable around 180200. That will give you the average error when somewhat high.
Do NOT use this procedure when you are around 40! While it would be nice to know, it is more important to get you back up ASAP! And, even if you did, your mental state is such that you would probably screw up the experiment anyway
This procedure will tell you fairly precisely what error you should expect in most normal conditions.
As a note, do make sure that your fingers are clean, but donâ€™t do your measurements any differently from how you normally fingerpoke, otherwise the results will not reflect your normal expectations.
Standard Deviation (SD)
If you want to calculate your BG meterâ€™s SD at each level, once you have measured the errors for a BG level:

square each error. The result is a positive number.

add the squares of the 15 errors for a given stable BG level.

divide by 15

take the square root of the result
What you get is the Standard deviation of [your finger + your BG meter + you] for the stable BG level in consideration