You're not going to get a "fixed position" no matter what

What we usually do for ground targeted AoEs is, first of all, run all slaves with their camera pointing downward. You shouldn't need to see anything else on your slaves under most circumstances, and this will both improve FPS and simplify AoE targeting. Then on a mapped key that casts the spell we activate Send Next Click, position the mouse, and click. I don't spam this type of spell on any of my teams because I want my spam key to be reliable; I AoE on demand.

Part of the reason you're not going to get a "fixed position" is that Blizzard has said that moving the mouse to a predetermined location is not allowed.

However, you can cut out the part about moving the mouse as long as it is *already positioned*. ISBoxer can pass a click without regards to the position of the mouse (with a Keystroke Action), so assuming you didn't leave the mouse positioned over part of the UI, such a click should pass through. Another reason, besides the sheer number of times your mouse will be over the UI, that this is probably not a good plan is that if your AoE spell is on cooldown you're still going to be sending this click, and likely changing your DK's target.

My recommendation is to do what I do, use a spammable DPS key but manually cast ground targeted AoEs, interrupts, and other special cases.