Knowing when to adjust the brake bias forward or backward depends on the specific characteristics of the race car, the race and the track conditions.

Common adjustments range from as little as 1-2% to around 5% in extreme cases.

When making brake bias adjustments, it's advisable to start with small changes and assess the impact on the car's handling and braking performance. It's important to have a systematic approach, making one adjustment at a time and evaluating the car's behavior before making further changes. Pay attention to the car's response during braking and cornering, observing changes in traction, stability, and overall balance. Driver feedback and telemetry data analysis can provide valuable insights into the effectiveness of the adjustments.
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Adjust Brake Bias Forward:

1. Understeer: If the car exhibits excessive understeer (front of the car doesn't respond well to steering inputs), moving the brake bias forward can help by increasing front brake force and improving front-end grip.
2. Wet Conditions: In slippery or wet conditions, shifting the brake bias forward can enhance stability and prevent rear wheel lock-up, reducing the risk of spinning out.
3. Front Tire Wear: If the front tires are wearing out faster than the rears, moving the brake bias forward can reduce front tire load during braking and alleviate excessive wear.

Adjust Brake Bias Backward:

1. Oversteer: If the car has a tendency to oversteer (rear end sliding out), adjusting the brake bias backward can help by reducing rear brake force and promoting more balanced handling.
2. Rear Tire Lock-up: If the rear wheels lock up easily under braking, moving the brake bias backward can alleviate this issue and improve overall braking performance.
3. Rear Tire Wear: If the rear tires are wearing out faster than the fronts, shifting the brake bias backward can reduce rear tire load during braking and balance tire wear.

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The telemetry for this technique is very subtle when looking purely at the brake pressure. It is dependant on whether you retain or lose traction from setting the brake bias more forward or backward. Therefore, look at the braking curve and see whether you are able to trail brake more or less depending on your settings.