Estimated size of table

Hi , I'm trying to estimate the size of table that will have 13 billion rows ( 13,308,848,738) in 3 years from now. The table structure is as follows

BEHAVIOR_SUMMARY_ID                                                                 NOT NULL NUMBER(12)
EVENT_CD                                                                            NOT NULL VARCHAR2(2)
INTENSITY_CD                                                                                 VARCHAR2(1)
ROAD_TYPE_CD                                                                                 VARCHAR2(1)
EVENTS_QTY                                                                                   NUMBER(6)
TRIP_SUMMARY_ID                                                                     NOT NULL NUMBER(12)
SYSTEM_CREATE_TS                                                                    NOT NULL TIMESTAMP(6)
SYSTEM_LAST_UPDATE_TS                                                               NOT NULL TIMESTAMP(6)

Presently the table has 10 records. After running dbms_stats I got average row length of 38. I multiplied 13,308,848,738 with 38 and dividing by 1048576 and I got 482,308 MB, and that corresponds to 471 GB. If I use Oracle's suggested procedure (create_table_cost) I get 1,050,304 MB and that correspond to 1025 GB. This gives more then double estimation as compared to the one used before. Could you please suggest what is the fight value, 471 GB or 1025 GB or something else ?