My buy box is ~505 to 520. Reason being, it's the next support level from the run up from 450 to 680 (see leftmost part of box) and it's fairly close to next big fib, also from the run up (76.4%). $500 is also a psychological level so it would be no surprise to stop several dollars before $500. I am charting bfx so there is a good chance of it going lower than stamp due to margin calls.