RG Live WotLK 3.3.5a

"The reason for this error message is that Thorns benefits from the druid's spell power at the time it was cast, not at the time of the hit. Thus, casting Thorns on yourself while at a higher spell damage (such as with a trinket activated) will make it impossible to refresh the buff without first right-clicking it off if your spell power has dropped down. Some buffs and items may temporarily increase spell power, causing this to occur. If using similar items or abilities with short cooldowns, it would be best to max out spell power before casting Thorns to get the maximum effect out of it. "
http://www.wowwiki.com/Thorns

Dornen sollte also nur während des Auftragens den dmg über zm berechnen.
Jedoch wird zz bei jedem hit die derzeitige zm des casters benutzt.
Dadurch kann man dornen nicht gezielt bei max zm proccs(gear) nutzen und so den höchsten dmg rauszuholen.(skalierung stimmt aber zumindest)