Okay so you wish to use the formula to pre-calculate number of fragments you would be requiring.
So in 1 segment maximum 480B of data you can send.
You have to send total 1800B of data.
So total number of segments would be Ceil(1800/480) = 4
You have made a mistake in the offset field. You should divide every number by 8 and then store.
0 480 960 1440 should be stored as 0 60 120 180