I have two ways to think of it, and both give similar numbers.
A: 250 years ago, 98% worked in farming. Today it's 2% (who produce more food!). Assume that the other 2% are at least twice as productive, and you get that 3% of the population now produces as much as 100% back then.
B: It's hard to directly estimate how much GDP per person has increased in 250 years. But the typical number economists get when trying is that it's 30x as big. Which means 3.3% of today's workforce produces as much (per person) as the whole workforce did back then.
Both A and B can be critiqued, but the precise numbers don't really matter for the argument.
A: 250 years ago, 98% worked in farming. Today it's 2% (who produce more food!). Assume that the other 2% are at least twice as productive, and you get that 3% of the population now produces as much as 100% back then.
B: It's hard to directly estimate how much GDP per person has increased in 250 years. But the typical number economists get when trying is that it's 30x as big. Which means 3.3% of today's workforce produces as much (per person) as the whole workforce did back then.
Both A and B can be critiqued, but the precise numbers don't really matter for the argument.