There is no optimal brake bias when on two wheels. In the wet it is best to rely mainly on the rear brake, as it's a lot harder to catch a sliding front wheel.
In the dry I mainly use the front.
When braking on the limit, it's a bit different.
On wet roads or loose surfaces I'll lock up the rear and judge how hard I can apply the front whilst keeping sufficient traction on it.
On dry roads I use the front so hard that the rear wheel is just about to start floating, then apply a minimal amount of rear brake to stabilize the back end.