Hong Kong via Beijing

Hi, we are planning a trip from London to Hong Kong with a 2 days stop in Beijing (time between arrival and departure flight is ~48 hours). When we return from HK to London we will have a layover in Beijing again but it will be only ~4 hours.
My understanding is that for the outbound journey we would be eligible for the 72 hours visa free transit and the return the normal 24 hours visa free transit is applicable. Is my understanding correct?
Thank you, Attila