@GhostSamFISHER well for a high-power RWD GT2RS, 0.39 biasfront will be palatable if it has enough downforce to take a corner at high speed - though honestly i would at least do over 0.4, but if it drives well enough for you then its one of the rare cases it will be fine.
And yeah I manually simulate tyre width differences thru tractionbiasfront