How to check dead zone of PFD?

How to run phase sweep between 2 identical frequencies going into the PFD, and plotted average integrated CP output current vs. phase difference to check to dead zone?
To eliminate the dead zone, we can add a delay circuit, and why the delay is choosed 5%~10% reference cycle? I think the delay time is ok if it is enougth to switch the charge pump, right?
Thanks!