FT: Ftl tugs, mass and psb question

I finally got around to reading the FT2, FB1 and FB2 books I bought at
Colours. I still need to do some design work for the fleet of the VRC, and
one of the things I wanted to look into was the ftl tug plus non-ftl
strikeboat concept. I intend to make everything FB2 compliant.

I have read through most of the list archive using the ftl and tug keywords,
so I understand most of the cost equations. However, I do have a few psb
questions.

The basic idea is to have a modular ship with an ftl core and non-ftl
strikeboats that fall off when the ftl journey is finished.

According to FB1 FTL costs 10% of the ships mass plus 20% of the mass of the
towed ships. This would mean(in the extreme): 1* 100 mass ftl core with: 10
mass hull integrity
                                10 mass ftl
80 mass extra ftl(400 mass capability)
4* 100 mass non-ftl strikeboat
Total mass is 500, with 90 mass used for ftl

However, with the same mass I could also do 4* 125 mass ftl strikeboat: 12.5
mass ftl Total mass is 500, with 50 mass used for ftl, leaving an extra 40
mass for extra goodies. Not all of this extra mass is free, as some of it has
to go into extra hull integrity and such, but some will be left over.

So, what's up with the psb for ftl tug mass costs? I can understand the 20%
rule if the tugged ships 'drift' inside the ftl bubble of the tug. However, I
see no reason why it should not be 10% if all the ships are mounted together.
I mean, the total mass is the same, so it shouldn't matter if that mass is one
big hulk, or a lot of bits that are clamped together.

I think I am missing something here, but I think I this is just elementary
physics. As far as ftl has anything to do with physics:-)

Cheers,