Less head-scratching, fewer mistakes
I could not find anything simpler than this. Straight from the OxfordDictionaries.com
Use 'fewer' if you’re referring to people or things in the plural (e.g. houses, newspapers, dogs, students, children). For example:
People these days are buying fewer newspapers.
Fewer students are opting to study science-related subjects.
Fewer than thirty children each year develop the disease.
Use 'less' when you’re referring to something that can’t be counted or doesn’t have a plural (e.g. money, air, time, music, rain). For example:
It’s a better job but they pay you less money.
People want to spend less time in traffic jams.
Ironically, when I’m on tour, I listen to less music.
'Less' is also used with numbers when they are on their own and with expressions of measurement or time, e.g.:
His weight fell from 18 stone to less than 12.
Their marriage lasted less than two years.
Heath Square is less than four miles away from Dublin city centre.
So, the sentences in concern:
There were no less than 50 people in the dining hall
In 25 words or less, please summarize what took place
Fewer calories (but 'less calories' has also ingrained in the language these days. Maybe, it's used where the number of calories is not specified. But still, don't consider this as a rule)
The hamburgers should contain no/not less than 50% meat.
Less than 5% of the population will be affected.
Further reading recommended here.
Worth noting that 'no fewer than' is an idiom which means the number you are describing is surprisingly large. Beware of using it that way!