Don't worry about the 5 second rate, it's the long term rate that matters most.
It is safe to experiment with the speeds so long as you have some cooling. Try from the lowest speed first, then build up and see what happens.
Give each setting plenty of time to stabilize before trying the next one. If you go above 2Gh you will probably need a powered hub to supply the extra current.
OK thanks, I've just restarted it at 0581. I'll run that for 24 hours to see if it brings the HW rate down, if it does I'll leave it at that until I've got a decent heat sink for it. I'm looking to salvage an old CPU heat sink or similar from somewhere and just get hold of some of those CPU pads as the go between
1 hour is probably enough, then up the clock and try the next one, then keep upping till the error rate shoots up.(then go back one step). Up to 2gh air cooling with a small fan is usually enough.
Above 2Gh then the power resistors need to be modified to reduce the error rate.