On Wed, Sep 16, 2015 at 9:27 PM, Jorge Timón <jtimon@jtimon.cc> wrote:

For enforcing new restrictions on your own blocks (thus at the policy level, not consensus) you don't need to wait for 75%. You can do it from the start (this way all miners setting the bit will enforce the new restrictions.

At 75%, you have a pretty solid super-majority.

You can safely reject blocks that have the bit set but are invalid according to the new rule (as long as everyone who sets the bit does it too).