I don't see why the reg is considered badly written.
Departure is defined as setting course overhead, or making an appropriate time adjustment as if setting course overhead.
So if we consider departure is 0800, then no alcohol can be consumed after 1200, AND, one must not have had so much to drink that one exceeds 0.02 BAC.
So, flight planning at 0700 is fine, as long as one's Ops Manual doesn't more restrictively mandate 8 hours prior to sign-on.