I`ve only shipped out China via HK in `big biz`, but usually if you are using a freight forwarder in HK, they can usually give you the information you are looking for. I`m assuming you are trucking your goods from Southern China to HK & then shipping by sea from HK. I`m also assuming you are buying FOB HK. (If not, disregard the below & private message me, as I might still be able to help). I recommend you to allow for the extra clearance time from China to HK, as I`ve had goods miss vessels in the past due to this. If you choose a freight forwarder & a vessel line, they can give you the details. Truth is, you might plan to use 1 vessel & due to timing, etc u might end up having to use another. I`ve even sent freight forwarders to factories to show them the most efficient way to pack pallets, containers, etc. (and truth be told, as someone who`s done biz in China for 15+ years, you can plan everything ideally, but when actual factory workers pack pallets & containers, it doesn`t always happen in the `ideal` way & efficiency is lost) We usually planned using `close as possible` estimates & then padded shipping costs by minimum 10%. Truth is, this is a `live and learn` `art` form. I used to get reports issued to me all the time w/ estimated vs actual shipping costs, and we updated accordingly & learned to figure them better each time.
You`ll want to be sure to count freight forwarder & broker fees into your costings if you haven`t already done so. Importing has been my life for the last 17 years...if you ever have any questions, please feel free to contact me.