Here's the formula I use:
steem_per_rshares = reward_balance / recent_claims;
used_power = parseInt(((votingpower * 100) * (vote_weight * 100)) / 10000);
used_power = parseInt((used_power +49) / 50);
rshares = parseInt(account_vesting_shares_available * used_power * 100);
rshares_convergent_linear = ( ( rshares + 2000000000000 ) * ( rshares + 2000000000000 ) - 2000000000000 * 2000000000000 ) / ( rshares + 8000000000000 );
vote_value_sbd = rshares_convergent_linear * steem_per_rshares * steem_price_sbd;