When can I buy with a reasonable assurance (confidence 7 on a scale of 1..10) that we're not due for a large (>20%) correction?
It is not knowable, any more than it's knowable where to put your chips on a roulette table with any level of confidence that it's not due for a 100% correction.
If you believe Bitcoin is going to strike it big in the long term, relatively speaking, it shouldn't matter much whether you get in at $10 or $20.