Database Size Estimation

Hi ,
I have a sample table with the following descriptions
CREATE TABLE "dbo"."tab1" (
"cola" "int" NOT NULL ,
"colb" varchar (30) NOT NULL ,
"colc" "int" NOT NULL ,
CONSTRAINT "PK___2__10" PRIMARY KEY CLUSTERED
(
"cola",
"colc"
)
)
GO
After adding some 100000 records to this table I run the sp_spaceused procedure and it give me the following information.
name - tab1
rows - 100000
reserved - 2122KB
data - 2084KB
index_size - 30KB
unused -8KB
Also i manullay calculated the number of datapages using the following equations which is available in the Sql server documentation.
Data row size = 8 ( Size of ffixed length columns) +
30 (Size of all variable length columns) +
1(Number of variable length columns ) +
7( over head)
= 46
Rows per page = (2032 -32)/46 = 43
32 - Page heade size
Number of data pages = 100000/43 = 2325 pages
= 2325 * 2 = 4650KB

Can any body tell me why there is a mismatch in result between the calculation?Am i wrong in some steps ? Is there any other way to estimate the size of table.Your valuable suggestions appreciated.
Regards
Jiji