im wondering how to set up this configuration

bandwidth -----> bh slave ---->bh master----->AP

if i wanted the bh to be configured for 75% download to the customer would i set the bh master to 25% downlink in the configuration or 75%?

yup you got it. the downlink percentage is the percent of data from BHM to BHS. So in your situation since you have the bandwidth at the slave and the users off of the BHM you would set the downlink percentage to 25% in order to push 75% of the bandwidth to your clients.