70 minutes should be fine, unless you hit a peak period in HKG transit. Peak periods tend to be in the morning, when Europe/Aus/US flights arrive with connecting pax onward to Asia; in the early afternoon (12-3pm) when the first batch of Asia flights arrive with connecting pax onward to Asia/US; and at night when Asia flights arrive with connecting pax onward to Europe/Aus/US. Using this as a guide, CX252 (7am) and CX250 (1pm) would arrive during peak times, CX254 and CX256 less so. But I am not sure whether there are any BKK connections from 254 and 256. If there are, that would be a good choice. If not, 70 mins is pushing it during peak hours, and sometimes the officers in transit security are less than pleased when you ask them to cut the line.