avail space

I'm a newbie to oracle (V6) and would like some help on checking space availability for my customer database. I;ve checked the FAQ sites with not much luck so please excuse me if this has been discussed.

I inherited the DBA responsiblity in my group so I'm learning as I go. What we have are tables of fixed sizes (no variable chars). The tablespaces are stored on raw partitions. Here are a few questions:

1. The raw partition block sizes do not match the oracle tablespace block size. For example, a tablespace of 450 MB is located on a partition which is 477 MB. Our oracle block size is 2k. The outgoing DBA mentioned the TBS's were created to fill the entire partition size. This does not seem to be the case since 450MB is less than 477MB. My questions are:
   • Does oracle require a certain amount of extra space in the partition for overhead? In this case the overhead is about 27 MB.
   • How is that overhead determined if its needed?
2. A table named LINE resides in tablespace TBSP1. The table size is 366 bytes as I discovered with select "USER_TAB_COLUMNS". I know the size of TBSP1 in oracle blocks. I know how many LINE rows (9592) there are. When I multiply the number of LINE entries (9592) by the LINE size (366 bytes) I get a number which should indicate how much space is USED as well as FREE in TBSP1. The questions are:
   • How can I verify the used space in TBSP1 using a query?
   • How much overhead is used in TBSP1 besides the 366 bytes size of each LINE row?

NOTE:
I tried using select "DBA_FREE_SPACE" but the value is related to EXTENTS. I'm not quite familiar with the differences between SEGMENTS and EXTENTS. However, the FREE segment or extent amounts don't add up to my manually calculated free space.

Thanks in advanced. Please respond here or via email if you can help.

-- 
Felix Cabral         {felixc_at_hogpa.att.com}      "Give me nukin winds,
AT&T Bell Labs       {  att!hogpa!felixc  }       a rad board, rad waves,
Holmdel,NJ           {    (908)949-1188   }       and a company sickday!"