The rule is pretty simple. The first 30 is: Count the number of seconds from the lighting strike until you hear the thunder. If it is less then 30 seconds, you are within a probable 6 mile range of the lightning. 6 miles may seem a long ways but keep in mind that a storm can travel extremely fast, faster then you could ever run...

The second part of the rule is to wait 30 minutes after you have heard seen and heard the last thunder / lightning sequence before commencing whatever your activity was.

In our case yesterday, the closest I counted was 28 seconds initially (twice) then more over 30 seconds as we took off for safety in the opposite direction of the storm which was moving to the northeast. Luckily this was a small and brief storm overall and did not come directly over us. What was really disconcerting though was how fast it appeared over the mountain range behind us...

You also need to ensure that whomever you are with, keep at least 20 feet distance from each other as lightning can easily hit one person then jump and hit other people who are too close.