Are there any tools to make it easier to BoP cars in the championship mode? Or do I have to test it through trial and error?
Dunno. When I used BOP, I did tested it. Tips I learnt from the experience:
-First of all, see what tyre the AI is using and if they change it from qualifying to race. Some cars do. You will have to erase that tyre with CM or even erase several tyres (maybe an AI uses supersoft on qualifying and hard on race) so they use the same on qualifying and race. Obviously you will have to make a copy of that car. You can try to balance the AI cars without erasing tyres, but it's not a good idea at all: qualifying will be useless because some AI will use a faster tyre and some dont and you will have to balance them when racing; which adds lots of complications (they will take a lot of time to do a clean lap).
-Some cars controlled by the AI pit no matter if you set 0 fuel and tyre consumption. Figure out that (with a 10 lap race must be enough).
-Use qualifying to balance the AI.
-Use a long track so the differences are less derteminant (1 second difference is not the same on Nordschleife and RB ring) with different type of sectors so the differences in time can be extrapolated to other tracks (a car very fast on Monza might be crap on the rest of the tracks). Spa is quite ideal: long enough and with 2 sectors that rewards speed and one for cornering (2º sector).
-I don't recommend to use longer tracks. The only advantage is that AI cars will have much less probabilities of ruining other AI's lap. But you will have to wait much more time (even with 100% grip, they take 2-3 laps to make a "perfect" lap). And if they mess up (traffic), they will have to do another lap. And in tracks Nordschleife that process can be endless.
-Use a Kunos tracks instead of mods for obvious reasons.
-In Spa, dont use more that 12-13 cars and less than 20 mins (so they have time to make a clean lap). And avoid, if possible, to use very different cars regarding lap times.
-I also recommend to duplicate or even triplicate the same car (12 AI would mean 4 different cars). You will have a better idea of the speed of the car: some AI drivers are faster than others (even with 0% AI variation). Because of this, you will see that with the same car and AI can be half a second faster than other.
-More obvious things: use standard conditions. This means 100% grip, no air or not random air, 1x time progression, 0% AI variation (very important)...
-Some mods wont work with max (or high) weight and air restrictor.
-Try to BOP the cars so they have more or less the same speed and cornering. Use the nature of every different sector: if a car is too fast on the straights but does well on the tricky sectors, use the air restrictor.