Hi James
Generally speaking, we would like the 'overall' spectacle power to be within +/- 0.25D as that is the margin of error in measurements. That is to say, differences of this amount are not considered significant.
When we say 'overall' power, that is because sometimes certain measurements may seem to show more astigmatism and less myopia, or the other way around. In such cases, if the 'spherical equivalent' that takes into account both myopia and astigmatism is essentially the same, the spectacle power may still be considered stable even if individual components seem to vary by a bit more.
Also, in another special situation, we worry less if a recent measurement shows less myopia than a previous one. This is because eyes generally do not become less myopic/shortsighted on their own. If an eye seems less shortsighted by eg 0.50D, it may have been that the previous measurement was 'overcorrecting' the real shortsightedness, therefore in fact the spectacle power is most likely to be stable.
At the end of the day, do discuss these concerns with the doctor performing your examination. As you can see from the above, the assessment as to whether a spectacle power is stable or not is sometimes not so straightforward.
