customizing disk: free sectors problem

Hello everyone!

I'm trying to customize tomsrtbt 2.0.103 to
add SMB support using the unpack.s/buildit.s
method described in FAQ question 7.

So, I unpack.s the diskette, and put in 2/bin
smbmount, smbmnt and smbumount files
and smbfs.o in 2/lib/modules. All the four files
were downoladed and bunzip2ed from tombsrtbt
addons webpage.

These files takes all together 725.328 bytes,
so I deleted some files that I don't need in
the diskette (in particular, nc, zcat, gzip,
gunzip, traceroute, telnet and wget binaries).
The deleted files free 767.711 bytes,
so the copied files should fit in the
diskette, aren't they?

...But when trying buildit.s I get an
ugly error message:

...
Cannot build, -439 sectors free!

I would like to know:

- What is the rule to calculate the
disk space that copying/deleting a
file takes?

- How can I free space to copy
the files that I need (in general,
any add-on that would be needed)?
Deleting more files?

Any help is welcome. Thanks very
much in advance!

--------
Fermín

PD. Sorry for the duplicated mail :(