I want to know what I should aim for when I tune the Parallel steer setting. It doesn't explain very well in the setup guide in the LFS Manual site, so I want to hear out what you think.
It's for XRG@BL1.

At 1.0 the front wheels stay parallel when you steer.
By lowering the value, the inside wheel will turn slightly more, giving a small toe-out value according to steering input => slightly less 'accurate', but can kill slightly understeer ...
As always, to be tested to find something that gives you a better feeling for the car (without speaking about performance issue, as it is not really a matter afterall).

Thanks for reply.
The steering becomes less accurate when I lower Parallel steer?
At this moment(I have +0.1 front toe), I found that 54% feels better than 27% perhaps because accuracy is not decreased like you say, but with 27% I did faster laps maybe because it has less understeer at entry. I'm not quite sure, but is it needed to change anti-roll bars in conjunction with Parallel steer to get a better handling, in order to get the accuracy of high parallel steer numbers while having the entry suffering?

That's why I asked what I should aim for when I tune parallel steer, because I don't know if I should look for the fastest setting while having a bad handling, or a good handling while sacrificing a small bit of speed. On the long term (endurance races), I think good handling is better, but in hotlapping it's not useful to have a slow set. I tried several settings but I always run into problems with each one. Around 10% car feels darty and way too oversteery in all corner phases. At 20%, the car is good in cornering but the entry is excessive. At 30%, it's like 20% but a little less darty and less oversteery at entry. At 40%, it's very close to 30% but less darty and less oversteery. At 50%, it's almost not darty but entry understeer is manifested heavily and a bit of mid corner understeering. I didn't try over 55%. The choice is vast but needs to be wise.

Doing the setup for a car is a whole combination of all the parameters.
Let's say parallel steer will help you to have the car behaving as you desire. Having the car behaving as you want can make it feels more accurate ... driving can be very subjective as everyone is not necessarily willing to have the same behaviour for the car in all the possible situations. For sure the general setup inputs (ARB, camber, tire pressure, springs, dampers) will play a higher role in the capability of your setup to be quick around a track.
Being quick in LFS is a big matter of practice combined with understanding what you are doing ... usually after some times building setup in "blind" mode, it is a good idea to take the setup of a fast guy, lapping a lot with it, and then to begin to modify the "fast" setup slightly in aim to see the differences brought by those changes ... after a nice amount of hours you begin to understand what makes you fast and what is not ...
Take note too that understeer and oversteer can be a nightmare as it is the case in RL : a slight understeer you may not notice may lead the car to a crazy oversteering behaviour when back on the throttle ... correcting the oversteering behaviour will then make the thing even worse => in the end you never correct the issue and you loose your trust in the car/yourself ...
As you may have already heard : practice makes better. I would add, without understanding (reading LFS manual is a good start), practice can be very long before making you better

i think it has to do with slip angles. The higher the load, the smaller the optimum slip angle. This means the inside tire needs more slip angle to be at its optimum.

you set the outside tire to the correct slip angle using the steering wheel but you need to set the inside tire's slip angle through the static (toe) and dynamic (ackermann) steering settings.

when you adjust the roll bars, you are changing the load on the inside tire and thus the optimum slip angle, so as Flotch said, everything is related and no setting can really be considered on its own.

25% xrg

The problem that the Ackermann angle solves is that when they wheels are parallel, the inside wheel is dragged sideways rather than rolling in the direction of the turn. This means the tyre is actually resisting the car's efforts to turn into the corner by providing friction in the opposite direction to the turn.

Increasing the angle (ie, lowering the value) points the inside wheel into the direction of the turn and decreases this friction. Overloading the angle will increase the sensitivity of your steering, making it easier to turn in as it is now dragging the tyre into the corner, but may cause the car to oversteer if the weight comes forward onto the front tyres suddenly. Likewise, underloading the angle can be used to correct cars that have a tendency to suddenly oversteer at high speeds.

Something to keep in mind: Either way you choose to adjust the angle, anything off the neutral angle will cause greater friction on the tyre when cornering, causing it to heat up and degrade quicker.