I don't use the setmsg function, but this is how i announce things;
If i meet a hostile looking ship, i ask it to halt. And tell them to stand by for a scan. If they seem to be fully hostile, i ask what their intentions are. If i get no response, i will ask again and add a line about me engaging them if i don't get an answer. If i still don't get a response, i'll open fire.
If i know the person is hostile, i'll ask them to leave as soon as they are within selecting range (9.9k). So that gives him/her some time to respond. If i notice the ship standing still, i will stop shooting. If the player appears to be AFK i'll continue shooting, since that's not my problem.
If i come across a group of players that are from the same faction or "race" as me and are fighting hostiles, i will cruise to them up until 6k. Then i use thrusters and pretend i am scanning the ships. Once i'm within the 4k barrier, and see the ships are hostile, i will announce that i am locking on a target, or activating weapons. Then i thrust closer and engage.
In my opinion, you don't have to write whole essays about your plans, as long as you give the other people time to respond. Being at 300m and suddenly say "engaging" doesn't give the opponent enough time to react.