There is no way to achieve 100% uptime on bone shield unless you have 100% avoidance.
Even at 99% avoidance there is a chance, however low, that the boss will hit you enough times to drop your shield.
The goal isn't 100% uptime anyway, what's particularly nice about bone shield is that it scales well with your avoidance. As you avoid more attacks, the uptime is greater, meaning that you take significantly less damage from the attacks that do land.
It is particularly useful for fights like Gormok where it can be up while you're tanking and recharge while the other tank has Gormok.