There’s an old joke that rules of thumb are useful if you’re measuring thumbs. And in that context I think Marnix's comments are very well taken, particularly his reminder that the carb barrel and venturi sizes are expressed as diameter, while their real flow effect is in circular area. In other words, not d, but pi r². So while the real delta may be 3mm, the smaller the diameter, the more % difference 3mm makes. Which means with a relatively small 29mm venturi, you might get by with a 32mm barrel – even if the ‘ideal’ rule is 1.25.