pctfree is the percentage which is reserved in a block for the updates of records in that particular block. This percentage is dependent on the nature of transactions, whether the table is read-only/has lots of updates etc.
Basically the steps to size a table are as follows:
1. Calculate the total block header size.

2. Calculate the available data space per data block.

3. Calculate the combined column lengths of the average row.

4. Calculate the total average row size.

5. Calculate the average number of rows that can fit in a data block.

6. Calculate the number of blocks and bytes required for the table.
For more info. visit [url]http://technet.oracle.com/docs/products/oracle8i/doc_index.htm.[/url]
Hope it will be helpful
manjunath