That depends on the usage of your retirement money. If you are the only one to use the retirement money then you need lesser cash but if you want to meet the expenses of your family then you obviously need more funds or BTC. Other than that, it also depends on the inflation rate of the country in which you are living. Because if I take my example, I need at least 3 BTC.

Just a hard guess maybe I would use less but If i came to know that i have less time now and the money i have remained then i obviously come up with a different plan like i will drop the idea of investing my money in anything.  The cashout plan is i will cash out only that which is required nothing less nothing more. This way i would be able to save more money.

Even if you would able to calculate now how much BTC you will be needed then you must have great confidence about the Price of BTC as we all know BTC is highly volatile and what happens if the amount of my 3 BTC or your calculated BTC becomes less over the period of time then you will be suffering too. So the best thing to follow is, do not calculate now, just make an estimate about your country's inflation rate and how you wanted to spend your money, and where you want to keep them.