I usually start with 850 MHz in "autotune" mode, after a couple of hours I use the "save current frequencies" to generate the required miner option. Something like:
--gc3355-freq=8D7830995748:850:0,8D7830995748:900:1,8D7830995748:875:2,8D7830995748:875:3,8D7830995748:875:4,6D7024984857:850:0,6D7024984857:875:1,6D7024984857:900:2,6D7024984857:875:3,6D7024984857:850:4,6D73109C4853:875:0,6D73109C4853:900:1,6D73109C4853:875:2,6D73109C4853:875:3,6D73109C4853:875:4,6D74216E4853:825:0,6D74216E4853:875:1,6D74216E4853:825:2,6D74216E4853:900:3,6D74216E4853:875:4,6D75227D4853:850:0,6D75227D4853:850:1,6D75227D4853:850:2,6D75227D4853:875:3,6D75227D4853:850:4,6D6E25A74857:875:0,6D6E25A74857:875:1,6D6E25A74857:850:2,6D6E25A74857:875:3,6D6E25A74857:875:4,6D75216D4853:875:0,6D75216D4853:875:1,6D75216D4853:875:2,6D75216D4853:875:3,6D75216D4853:875:4,8D7230965748:875:0,8D7230965748:850:1,8D7230965748:850:2,8D7230965748:900:3,8D7230965748:875:4,6D7838894857:875:0,6D7838894857:875:1,6D7838894857:875:2,6D7838894857:875:3,6D7838894857:850:4,6D7635624857:875:0,6D7635624857:875:1,6D7635624857:875:2,6D7635624857:875:3,6D7635624857:875:4
and finally I disable autotuning. As a result I've a very low HW-Error rate and devices already start with "best" option.