you answered yourself basically "trial & error"
first start with your ram increase 5mhz at a time then benchmark.if you see no artifacts or strange objects its ok.
just keep increasing 5mhz at a time then bench in that order when you start seeing objects on screen that should not be there go down to the point things were fine then leave at that.
then the same goes for the gpu clock. but general rule is go a little at a time.
people have different o/c rates that is because of cooling.good gpu heatsink & ram sinks etc.