I would use windows for initial setup at least.  Only modify straps, don't modify anything else in bios.  Run miner.  Use overdrive or afterburner to increase memory clock until you either get memory errors in hwinfo or the hash rate stops increasing, then lower the mclock a bit.  Lower cclock until hashrate decreases.  Last, lower core voltage as much as possible while maintaining stability, then increase it a little.  Sometimes modifying the core voltage automatically modifies the memory voltage too.  If not, then lower memory voltage as low as you can while maintaining stability (check for 0 memory errors), then increase it a little.

Once you've determined the proper settings, you can use them in linux by either programming them into the bios, or setting them in the claymore command line.  There may be other ways, I don't mine eth on linux.

If you don't lower the voltage, your rig will draw way too much power.  Properly undervolted, an 8x rx470 rig shouldn't require more than 900W if mining only eth.

You may not be able to push them to 30 mhs without custom straps, although 29 is usually doable.  But you need to accept whatever hashrate you get that is stable, even if it is 28-29.  Not all cards tune right up to 30 without custom straps.  And even ones that do, they may not be stable, and it will probably cost you more in the long-run to push your cards to their absolute limits.

In general, people who are relatively new and don't entirely know what they're doing expect way too much and push their cards too hard.  If you don't really know what you're doing, and instead are just following guides/instructions online, you should play it safe and be conservative.  Under those circumstances, 230 mhs at 900W, and stable for weeks or even months, should be doable.