The default 5,5 appears to be "stock" brake balance. So if the car has a 70% forward bias stock, setting the in-game "bias" to 5,3 reduces overall power by 20% and shifts the bias forward to 79.5%.
The "formula", should you happen to know the stock bias (and it's correct in the game), is:
B*F/5 / (B*F/5 + B'*R/5)
Where
B is forward bias,
B' it's complement (rearward bias),
F and
R are the bias settings in game.
For
70% stock bias,
5,3 setting:
70*(
5/5) / (
70*(
5/5) + (100-
70)*(
3/5)) = 0.795..., i.e.
~79.5%
For
70% stock bias,
5,7 setting:
70*(
5/5) / (
70*(
5/5) + (100-
70)*(
7/5)) = 0.625, i.e.
62.5%
For
62% stock bias,
4,2 setting:
62*(
4/5) / (
62*(
4/5) + (100-
62)*(
2/5)) = 0.765..., i.e.
76.5%
So depending on whether you want a more or less forward bias than stock, sometimes you need to set the rears "stronger" than the fronts.
The bias setting does nothing outside of a race, although I'm not even sure it works online at all, so although trial and error is still king, there's not much provision for it in the game, whereas it always worked in GT5.
Is this another thing where PD think they're "balancing" online by removing the ability to adjust the bias? Seems to me if people are disabling ABS, they're going to expect that bias settings will be tuned to personal taste, and accept that they may be slower than others as a result.
Really, the existing adjustment needs to be replaced with a proper, transparent bias and overall force adjustment instead of this silly use of low-integer rational numbers nonsense. Then allow that adjustment everywhere in the game.