General wisdom seems to be that most people in ketosis will be between 1/3 and 2/3 of the way down the colour chart but the actual colour may vary according to make of stix, dilution of wee at the time you do the test, personal metabolism etc etc etc.
I seem to throw out a lot of ketones. My stix when I bother to use them are usually a deep plummy colour.
Different people react to dietary ketosis in different ways. Ketostix only measure the excess ketones excreted, and not ketones produced. Some people metabolise more ketones than others, and therefore those which are excess (and available to be measured) vary from person to person, tolerance to ketones, the amount of exercise undertaken, the time of the day, water intake and probably the conjunction of the planets. Some people can be in ketosis and may not register on a stick at all.
Therefore ketostix are only relevant in detecting excess ketones and they're good for telling you that ketones are there if you're not entirely sure. However (unless you're diabetic and are using them for diagnostic purposes) the scale is unfortunately not an accurate measurement of anything for dietary purposes.