Here is the formula, figure out what your realistic annual expenses would be better to overestimate then underestimate here. For example $50,000 x 25 = $1,250,000 which at current prices is about 12.5 BTC. Annually you liquidate 4% of your stack for expenses the remaining 96% will continue to grow. You should be able to withdraw 4%. That said with the drawdowns you probably want to hit this number at the bottom of a bear market. The delta between peak and bottom can be huge with bitcoin as you know.