How to calculate number of rows per page in postgresql

Hello,

             
The following is the procedure to calculate the disk space occupied by
postgresql from a flat file.

 http://www.postgresql.org/docs/faqs.FAQ.html#item4.5

In this I didn't understood some terms 

24 bytes: each row header (approximate)
24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
what is that first 24 bytes and last 4 bytes represents?

I did practically but for a  table with two integer columns it
takes 185 rows for a page size of 8192 for 186th it changes to
16384.But when I applied this procedure.......

     8 bytes: each row header

     8 bytes : two int fields     

   + 4 bytes: pointer on page to tuple   

  total 20 bytes

  8192/20 =410 rows per page (approx.)

     Can you please tell me how to caluculate number of rows per page size.

Thank You,

Avin.