checkshots upload

hallo all,

I have Petrel 2011.2 version.
I am trying to upload checkshots for a well and i am not sure how it is supposed to be written.
my first value is 0 (twt) correlating to 12.85 (MD). how do i know if 12.85 means below seafloor or above seafloor. my kelly bushing is 12.65 m. And do i need to upload the checkshots in negative or positive signs?
when i upload the sonic log and the checkshots to the calibration window i get an error message that the checkshot zero time is not at the elevation of checkshot tz datum. what should i do?
I am a bit confused...
Thanks a lot!