The rule of thumb is that…

我想应该翻译成：大体的原则是…

The rule of thumb is to wash your whites and colours separately.

根据经验，白色衣服和其它颜色的衣服应该分开来洗。

The rule of thumb for improving in anylanguage is simple practice.

提高任何语言能力都有一条经验法则，就是多练。

The rule of thumb has been red wine with meat, white wine with fish.

“吃肉时喝红酒，吃鱼时喝白酒”也确实是经验之谈。

The rule of thumb is to drink six glasses of water for every glass of alcohol.

经验法则是喝1杯酒要喝6杯水。