What does it increase with each increase of the armor value? I might be able to make something to reverse it if I can find a pattern, so it would be good if you can post a few example values (real ones, ofcourse).

That sure as hell look like Float or Double to me. What program are you using for editing the .sol? Have you tried using a HEX editor? When I used to edit .sol the values were usually assigned under Hex as 3FF0, 3FF8, 4000, and so on. That's how float and double are represented. Later on, it converts the values to w/e the game maker has established.
You could always read more of it on the almighty Wikipedia for more info on how it works <http://en.wikipedia.org/wiki/Double-precision_floating-point_format#Double-precision_examples>.

Now about the possibilities of adding another effect on the weapon and of the list of effects.That depends on how the game reads the "effects" of weapons. How does it read? Is it read as "Perfect" weapon? If so, then it would not be possible, because a perfect weapon would have a pre-set effect list, and as such, you would have to modify the game engine to be able to add another kind of effects. If it's read as attributes, then it might be possible to modify one of the attributes that a weapon has with another. Just modifying the .SOL and make it have more effects would also be possible if it has the attributes listed one by one at the same time as it doesn't have a set limit of how many attributes a weapon can have, associated or not with the quality of the weapon. You'd have to try to see if it's possible or not trying.

Well, I have quite a few programs that can edit .SOL files, but first of all, I'd reccomend you to go to our so-very-useful Resource Section and look at the topic of Useful Programs. Minerva is quite old, I mean, QUITE, and has several limitations, so replacing it with a more up-to-date and better program will make your hacking life a lot easier.
Second, I have it opened on 010 Editor and I find it somewhat hard to follow you as we talk about since they are 2 very different types of editors but I'm trying to follow you up. I have played the game before, but still, I didn't try to hack the .SOL since I went straight to a AoB, so right now I'm kind of lost, meh :p.
I noticed that it does have a very simple way of addressing to the positive effects, as well as negative, but since you can't do it on Minerva, try doing it on another program.
P.S: Let me just find a game that I find it's worth hacking and I'll submit it for you :p

You can do that, that's for sure, but there are other ways of doing the samething. If you know how the .SOL stores the value, you can change it to something else. You could always try to change "rangeMore" to "damageMore" and see if the game accepts. It will accept if there's no verification, like a hashtag or w/e else people would put it in the .SOL to make sure they don't hack it.
There are a few games that do that, either encrypt the .SOL or that verify the content of the .SOL with a checksum, but on this case, I don't see that anywhere, so it's more likely that there's no such thing here.
@TNC: People don't do that because right here, on KongHack, people would rather make a AoB than temper with the .SOL. Tampering with the way a game works offers a lot more option than tampering with the .SOL, since doing so just allow you to get a somewhat unfair advantage in the game. But it does have it's benefits as well, so it's also a valid hack atempt I guess.

It isn't that hard to use a Hex Editor. I, personally, would rather use a Hex Editor than a .SOL editor. It's a lot more powerful, in that you can add new things as well as change them, though, as you said, it does require more knowledge than using a .SOL.
I might write one in the future, as soon as I finish writting the one tut I'm writting right now. Just 2 chapters to go, and finish writting about how to paste .SOLs...
The Hex Editor I was used to use was HexWorkshop Pro. It is paid, but I used it for a long long time. Right now I'm testing Kadat's Choice, 010 Editor. You should try doing it at some time. It let's you do a lot more than you can do with a .SOL Editor. The .SOL I use, however, is the actual .SOL Editor, which, as TNC said, is severally outdated, and won't work all all the time.

It's nice to have a HEX Editor. I began using those before using .SOL ones, so I'm a lot more used to them.
Also, they're nice because you can not only open SOLs, but also other save files, and just about anything you have in mind. :p (Go and hack that annoying pc game, or xbox360 game save lol)

92Garfield made one: http://forum.cheatengine.org/viewtopic.php?t=515329

I haven't tried it much so I don't know if it's good.