As a long time weather observer in polar climes, and an aviation weather qa specialist, i would add that the etages are not a fixed thing, per say. They do compress (lower) towards the poles. For example, you may find what looks like stereotypical CI (cirrus) at 150 AGL at the poles, or AC Lenticularis (Altocumulus that is sheared almond shape usually by terrain and high resulting wind) at levels as low as 040 AGL, which technically makes them SC (Stratocumulus) (rotor clouds?). We would just put a remark in if it doesn't fit in the computer ;) There is only one resource that is authoritative in your country, and it is written within the directive provided by WMO. In Canada. we use this:
https://www.ec.gc.ca/manobs/73BC3152-E142-4AEE-AC7D-CF30DAFF9F70/MANOBS_7E-A19_Eng_web.pdf this is the latest edition to date and legal in Canada, but may be a good guide for you also (I don't know what Country you reside in). Very clear cut etages were given in the past, but WMO has moved away from this as of MANOBS 18. The etages (the old term for heights) used to be extremely rigid. Now they are considered a guideline, acknowledging that the clouds and etages are both variable dynamic things, not something that can be set in stone (anymore). -And just to add, cloud heights are generally given as height above ground level (AGL), not ASL (above sea level) unless you happen to be observing them actually at sea level. Aircraft will report heights ASL, but the observer has to apply a correction (if report was given over the station) before publishing. I suppose there are some places where the height is below sea level, but I have no experience with this. Hope this helps!