Specific question regarding time frames of 72 hr visa.

We arrive at 12:25 PM on the 15th in Hong Kong, and our ticket is at 12:45 on the 18th, making it for 72 hours and 20 minutes hours in transit. Does anyone know how strict the Chinese government is on the 72 hour rule, and with overlaps (customs leaving the airport, and entering before) does the time start when you enter and leave customs?

Thanks!