Why not the whole of your capital? If you are sure that the risk in investing in BTC is minimal then it is a good move to invest 100% of our capital. Then we can just save another one to venture for another investment.
In anything you do always think of the negative outcome even if the chance of success lies at 75 -90% anything can happen, so I don't really consider this as a good advice .
It's normal to add a larger percentage but considering everything isn't a good one for me though ,Expectations are limited to some extent so always have a backup plans for all.... no matter what