Synopsis:
"As a result, eating two cups of ground apple seeds might be fatal."
"...you would probably need to thoroughly chew and swallow over 150 seeds before you experienced any adverse symptoms."
But there's around 28x variability for toxicity: 203-5,670 apple seeds to get a lethal dose for a 150# person. That's a lot of guesswork. I suppose the easy approach is to just go for the maximum dose, but given the genetics of apples --they have insane variability-- it still sounds chancy.
All that said, it does give credence to the "urban legend" I once heard of someone accidentally killing himself by hearing of apple seed toxicity, poo-poohing it, collecting apple seeds, and eating them --and discovering he shouldn't have been so skeptical.
I cannot help but point out that an apple a day may keep the doctor away, but in about ten months you should have enough apple seeds to bring the coroner on the run.