Sql Server page structure. What is Fdata length?

I am trying to understand the structure of the Sql Server data pages. This is the screenshot from the Pro SQL Server Internals by Dmitri Korotkevitch I've created 3 tables:

1. 1 INT Column
2. 2 INT Columns
3. 4 INT Columns

All columns are NOT NULL

Then I run

dbcc traceon(3604);
dbcc page
(
'DbName' 
,1 /*File ID*/
,368 /*Page ID*/
,3 /*Output mode: 3 - display page header and row details */
);

and getting following values for Fdata length:

1. With 1 Column - 0800 = 0008 = 8 = 4 + 1x4
2. With 2 Columns - 0c00 = 00c0 = 12 = 4 + 2x4
3. With 4 Columns - 1400 = 0014 = 20 = 4 + 4x4

Here I've listed "value in the output" = "swapped value" = "decimal value"

EDITED: So far as I understand it is Const_4 + Nbr_of_Columns * Size_Of_Columns. What is this Const_4?