Yeah I would use something around 10mm thick for flooring as a minimum.. and dont forget to dig a big fat square hole for the pier... deeper the better in most cases.. just make sure you dont hit any underground pipes or the like.
1.3 - 1.4m deep should be stable enough for any gear you would want to mount on top...
Or obviously just use your tripod, but make sure the area under the mounts legs are supported well...