In a field of 25 or so cars in a 10 lap / 15 minute race you can expect a dirty [real] race to have 2 or 3 yellow flags. That's 250 laps and 2 to 3 yellow flags: 1%.
The racing I saw this weekend, some of it was apalling as some drivers couldnt stay on the track on the warmup lap, and yet they still managed to keep it under 1%.
Because our average is 28% even the good guys get caught up in things a little too often [in avoidance] so our best racers still have around 4-8% or so, as they're avoiding the drivers causing so many yellow flags. Our system is also way more sensitive, but I think 10% is a reasonable target to aim for.
One of the problems with sim racing is that it really is no big issue to have a spin or two, the next race is only 10 minutes away. We drive closer to the limit, and we have a wider variation of driver skill on track, so we get more incidents and hense more yellow flags.
If I could find a way to change this then I will, hense you can score a bonus for getting a good yellow flag ratio in a given race with 100% being well rewarded.
I've set the bar high because I want the system to encourage an overall standard of improvement. Sim racing still has a way to go.