The slide freq bar is frustrating as it only does 5 mhz increments. Not good when blades sweet spot is 838. --freq=838 does not override the slide bar on save. You have to:
nano /var/www/minera/conf/miner_conf.json
and fix by hand.
Another option is to save frequencies on dashboard @800. Then go to Settings, copy whole set of frequencies (starting with --gc3355-freq=48FB50683831:800:0,48FB50683831:800:1,48FB50683831:800:2...) and paste it to notepad. Use Replace all ":800" with ":838" and copy+paste it back to Extra command line.