[Space] How habitable is my universe?

[balzacq]

I have a PHP script to generate systems as per GURPS Space, with a very few minor tweaks (Agemegos' revised tidal formulae and aging the system in 0.1Gy blocks to track changes over stellar evolutionary timescales). I use the chart in G:S to generate stellar types instead of real-universe data. Also, I use all the rules except for brown dwarf and flare star generation.

I've generated a total of 16977 systems this way (I have a perl script that creates whole thousand-system sectors at once). The habitability breakdown is as follows:

Hab.   Systems    %
8  	136  	0.80
7 	300 	1.77
6 	257 	1.51
5 	338 	1.99
4 	303 	1.78
3 	180 	1.06
2 	105 	0.62
1 	28 	0.16
0 	15317 	90.22
NULL 	13 	0.08

(Without diving into the tables right now, I suspect that the NULL values are from systems with nothing but gas giants and belts.)

I'm at work and without my books, but I was under the impression that you didn't even generate carrying capacity for Hab 0 planets -- the assumption was that Hab 0 systems would only ever have outpost-type residents (miners, military, etc.).