Flytdeck, sorry mate, but this is bollox:
Not a rule of thumb, but a CARDINAL rule. If a commercial aircraft lands with less than 30 minutes worth of fuel, then the flight crew has not done their job properly.
Look, fuel loading at departure is a planning exercise. Take all available information, check it out with the flight plan, take a sensible amount of fuel to achieve task, complying with all national and company regulations but still economically sensible and launch.
En -route things happen which cause changes to the plan, but that's ok because we have dispatched legally and we can update the plan as we go. We can burn contingency if necessary, we can also burn diversion fuel if we meet the requirements to do so.
So despite getting to destination with less than planned, we are still legal, we plan to land with more than reserve (30mins) fuel.
Due to further unforseen circumstances (say G/A at destination due previous aircraft slow to vacate) we actually land with 29 minutes fuel remaining having made all the right radio calls and informed all the required people.
JOB DONE!