Hong kong to Guangzhou and back

Hi,
I will go to HK soon and would like to travel to Guangzhou for 3days and visit the different markets than back to HK. Will I be able to do that with the free 72h visa? and since I will live at a hotel there is from what I understand no need to visit a police station?

And If I understand the rules correctly I could fly in to Guangzhou at 9am on the 5th and the 72hours start on the 6th? so that will basically give my almost 4 days there on the 72h free transit?