It always splits by 50/50 based on steem code below.based on my research, what @lennstar said is correct.
auto sbd_steem = ( author_tokens * comment.percent_steem_dollars ) / ( 2 * STEEMIT_100_PERCENT ) ;
it's maximum 50% when percent_steem_dollars is 100%.
I couldn't find the place where the portion of SBD is adjusted by the bias.
What is your source that made you think bias adjusts payout ratio?
I wonder if i am missing something.