Booking a train from Changsha to Shenzhen

I am taking a bus 8am from Fenghuang to Changsha (I am not sure what station the bus drops me off at in Changsha, and I do not know the exact time. Maybe 5.5 hours?) and I am going to book a train ticket from Changsha South Railway Station to Shenzhen North Station. Since I do not have an exact time of arrival in Changsha, and I must take a taxi from the bus station to the train station in Changsha, what train number should I book? I do not want to leave too late since I need to go to a hotel in Hong Kong after I get to Shenzhen and I do not want to be held up at the border too long at Futian. I also do not want to wait too long in Changsha.