WL1837MOD: Calibrator tool to verify 5GHz functionality

Part Number:WL1837MOD

I am using the calibrator commands to put the module into a particular mode of operation, as described here: http://processors.wiki.ti.com/index.php/WL18xx_TX_Testing

For testing 802.11a band, I set the channel using: calibrator wlan0 wl18xx_plt tune_channel 36 1 1

I am unsure how to set the transmission rate (for setting data rate and modulation scheme), to correspond to a particular 5GHz channel. For example, if channel 36 corresponds to a data rate of 6 Mbps, then why shouldn't I set rate =4?

Thanks