not only have I given serious thought to your question as it pertains to my eligibility for the ACA PTC, but I have run the #s and moved $s around to try and ensure I stay below the cutoff. With regard to how much "headroom," well that depends on one's circumstances. I would prefer targeting a mAGI at least a few thousand $s below the cutoff, but I have so many moving parts in my financial life (including a spouse whose 2016 income is currently a mystery), that even a 3K headroom may or may not be enough. I left ~1K open in my 2016 tax-deferred space, so in April 2017, if I come out 1K above the cutoff, I can pull the trigger on an additional 1K shift to lower mAGI.
But to specifically answer your question, I targeted a 1K "headroom" for ACA PTC qualification. I'd prefer much more to feel comfortable.