The historical "average" decline would be larger, around 70-75% - note that I'm not saying "normal", since some fall less and some fall much more. Some fall all the way to $0, which skews the distribution to the left.

If you wanted to apply historical examples to get a range, you'd start where the rally began ($250 or so, early 2015) and watch for where parabolic growth began - about $1000 at the beginning of 2017. That first $750 is "safe". Say if BTC were to top out at $11000 (very unlikely, but the math is easier this way), somewhere between 50% and 80% of the 10K rally could be lost - down to the $3000 to $6000 range.

But you can't call the top - so you see the problem with trying to apply even a rudimentary calculation. What if the near-term top is $15K? $20K? $50K? They all present very different pictures coming back down.