On main event, add a new event "Health Change", press the red crosses to delete the two default event conditions. Add in the following two:
Condition: "Unit ID"
Relation: "Is equal to"
Relation: "Is less than"
That will now alert you to when you can dispatch below 35%, I also add in an exception to not alert me if it's already done so a few seconds ago. This is useful for when you have many targets around and dont want it spamming you each time you target a new one.
I also add a subtle tone to the event trigger as well, this is done easily by finding the trigger in your list of triggers and pressing the screwdriver and spanner icon and looking under sound.