It's actually not that simple <Snip>
True, but close enough.
Just 5 steps?
Or more, or less. To me it's not important if or how much characters can increase their characteristics. What matters is that it doesn't make the system go all screwy.
Maybe it would be an idea to create scaling within the scale, so to speak. Say, S30-50 starts at 30Kg with every S point increasing carrying capacity by 1Kg. Then 51 might increase carrying capacity by +100Kg, with every point afterwards increasing it by +10Kg. Or.. You know, something along those lines. Do note I've given the exact values about as much thought as it took me to write this.
Additionally, it might be a really good idea to introduce a species maximum on characteristics values. Off-hand, I think the stuff in the wargame can't go more than 2 points over the species base values.