SRo AC FTE clarification

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Just looking for some quick clarification on the FTE rules as they relate to the AC in SRo, per a scenario that occurred about an hour ago. This felt a little "gray" to me, so I'd like to know for sure, should something like this ever happen again.

AC popped, and the closest person to him didn't realize it, as he was on the other side of a dune. Being a DRU, I got track up, threw up Spirit of the Cheetah, and took off after him. A third individual must have seen me take off like lightning and made chase. I made it within casting range and began... or so I thought... to cast Engorging Roots. Unfortunately, my mouse has been acting up recently, and the associated hotkey doesn't always fire unless firmly pressed. In the heat of the moment, I had completely forgotten about this, and only realized I wasn't actually casting after a good second or so. Tried again, failed again. Third time was the charm, though!

At this point I had been visibly standing in front of AC for a good 2 seconds, and visibly casting for around half of that. Obviously attempting to engage. The third individual shows up with a Wand of Frost Bolts (instacast nuke) and manages to get a clicky off about half a second before my cast completed.

My impression of the rules are that first damage is more important than obvious intent, at least as it pertains to a randomspawn like the AC. Is this correct, or should I have attempted to contest the spawn? I was fortunate enough to snag a kill yesterday around this time as well to finally get a buddy of mine a pair of Jboots, so I was content in just letting this one go without much of a fuss (although I was quick to let the guy know how fortunate he was that Razer is my brand of choice in gaming peripherals. Maybe not for too much longer.)