Well actually I also thought about this idea myself, I must add to it actually that I own a Razer Mamba as mouse for my pc which allows macro recording within the driver program. If you do not have anything like this (keyboard or mouse), it's gonna be harder but there are ways. Anyway, I will explain what I would do with my mouse as macro.
1. get a stopwatch
2. go to the mob, AND the spot where you want to activate your macro
3. start attacking the mob, and simultaniously start your stopwatch.
4. DON'T use any skills, as you can barely configure those in macro due to cooldown times
5. once the mob is dead, you know how many seconds it took you to kill it, add about 3-5 to this depending on how fast you know the mob respawns and how many there are at the spot.
6. when configuring your macro use : tab press->tab release (without any delay) then space press->space release (again without delay) and last but also most important, add you time in seconds in the end, this is to make sure your char doesnt constantly select the next target but keeps autoshooting until its dead. (or any default unit your way accepts, for me it is miliseconds, so I need to multiply by 1000)
7. a few ways that might help timing are: keep timing until you kill the mob WiTHOUT any criticals; DONT choose a spot which is anywhere near a boss, you would not want to choose lake ghosts for example, you dont want to be attacking aquarior or even serbetus all of sudden XD
and one last thing: when you use this method
I am NOT responsible if you get banned for botting, because this is actually botting and NOT allowed!!!