NOTE: These are my recommended settings for you that you might try out.
NOTE #2: Description in slightly more details can be viewed below the following picture.

What's the overall effects strength for ?
As it speaks for itself, it's for the overall strength for the FFB. You may lower it, but keep in mind that the FFB won't be that responsive if you lower it and if in case you add more than 105 percent, it might leave a slight damage for the wheel in near future.
Spring effect strength.
Plain and simple for this one. Adds useless shaking effects, I guess. lol
Damper effect strength.
I recommend NOT to add even a bloody 1 percent to this thing. It should be rather named Useless Effect Strength. It adds completely useless effects to the wheel and can torture your FFB really badly.
Centering effect strength.
Any added percents will annoy you more during a race
Degrees of rotation.
Complete preference. If you like to have 540 degrees of rotation, then feel free to do so, sir/madam.

This thread is going to have a slight update in the near future.

had the same setting on g25 @ 720.

You guys forget to mention LFS FFB settings, known as FFB presentage. Without it, your 105% overall effects strenght, does not have any meaning.

MousemanLV : if you plan to make some hardware manual in the future, please do your homework and learn what each setting segment does, before you start to make fun of it.

LFS is the game with the best FFB effects, so you can turn all additional effects in logitech profiler off, because LFS covers them all. But, these settings can come very handy, for playing other car games (not simulations), that have pretty unrealistic FFB. Names of these additional effects, pretty much explain them selves, but exact value for each one, you must find yourself by trial and error method. We're not all the same and we want different things from FFB effects, so that's why logitech guys, made these settings for us in profiler.

Overall effects strenght should be at exactly 100%, beacause your effects will be linear through the whole range of rotation. For me, I preffer 720 degrees of rotation for all cars, since I think (after a month of hard trying) 900 is just too much (unless you set FFB off). In LFS, I use (7-17)% FFB strenght, depending on a car. I use dfgt wheel.

I already mentioned that degrees of rotation are a frkin preference, lolololololololololololololol.

Overall effects should be at 100 percent ? Duuud, max amount that u can put there is 150, great maths dawg.
@ 100 percent ? pls, it cant even go up to 200 percent, so whats so LINEAR about it ? It's bloody measured in percents, regarding how much of le effects u want, measured in bloody percents.

Since the motor in the wheel can only provide a given amount of physical torque, won't you clip the force feedback peaks with that much feedback? I'm just thinking if you want the full range of force, you have to adjust the maximum force according the highest peaks of feedback from the game, even though that means low torque forces in most racing conditions.

http://en.lfsmanual.net/wiki/Wheels#Logitech_G27

Last topic. I always stuck by these recommendations but never knowing if they are correct.

@Skagen
Yes your line of thought is correct.

It seems that LFS itself clips forces (same as it clips audio, but that's another story...) if they are set anything above about 25-30, so it's best to stay at this level in LFS to get a full range. This was investigated by peoples on LFS forum some time ago, can't remember who.
This means to be able to change your overall force strength, you can only increase the Windows controller settings as Mouseman has shown.
I'm not sure but I think something slightly odd does happen when you go over 100%, though I'm not sure what. You can be sure though, that setting 100% in LFS will give you clipping.

Short story is, as with a lot of things, don't assume that because something looks a certain way, it works that way.

Personally, I use 25% ingame & 110% in controller settings, or something like that, with G27. Oh, and 360 rotation, because 'it just feels right'

When I used to use the legendary red Momo wheel, the FF was just great with that, but the single motor very weak overall compared with the current wheels. To get decent force you had to have LFS setting up at 80% or so, and there definitely was clipping with that, beyond about 10 degrees turn, the force was just constant. Still, while you were only turning 10 degrees, it felt great!
The G25/G27 still don't feels as good as that did, but they now have the power to be able to give reasonable force, so LFS setting can be down in it's 'good' range, less than 30%.

A proper wheel could do with a real proper motor in it. Those in the G27 are still not big enough - they just get too hot & do not last. An electric scooter motor, belt driven G27 mod is on my project list...

PS:I hope that the steering wheel does not break the time someone hands in traffic accident during the race