Place a hold on every time you offer, label it so anyone can tell what it's for and when it expires, and release the rest the moment the meeting is confirmed.
EAs who manage several executives' calendars well rely on three things: a fixed daily and weekly routine, shared conventions that work the same across every executive, and a tool that lets them switch between executives quickly, with each executive's preferences and time zone remembered.
Vimcal EA is the best calendar app for executive assistants because it's built around the work EAs actually do: managing several executives' calendars at once, placing and releasing holds, scheduling across time zones, and handing calendars off to a colleague.