When you come to servicing a modern watch, chances are you will have access to the technical information for that specific calibre. In the information pack you will find the watch’s reference FREQUENCY. This is the frequency at which the hairspring is expected to operate in order to output the correct time. A variation from the reference frequency will manifest in either a gain or a loss. Given the fact that a hairspring is a physical object, we can expect certain imperfections to exist in its structure, meaning we should expect a fluctuation in frequency, which for a 21,600 train count should be 3Hz (21,600 vph/60 minutes in an hour/60 seconds in a minute = 6 vibrations per second. 1 hertz is equal to one oscillation (twice a vibration) and therefore you divide 6 by 2 to reach 3 hertz).
So if, for example, you expect your watch to operate at 3Hz but, upon placing it on the timing machine you discover the frequency VARIES between 2.995Hz and 3.005Hz, how can you expect this to manifest in timekeeping errors?
This example is unrealistically straightforward, as the +/- variations of the frequency are exactly the same. Normally, you would have to do the following calculation twice, but in this case once is enough. The result of this calcutlation will show you the potential error in the daily rate.
Gain/loss = 0.005 either way.
0.005 x 3,600 (seconds per hour) x 24 (hours in a day) / 3 (ref frequency)