According to a booklet I have (The Complete Steam Locomotive Companion by Fletcher Cox), N&W 611 weighed 494,000 lbs with a fully-loaded tender of 378,600 lbs, for a total of 872,600 lbs.
Using the above formula, that comes out to approx 8 lbs in 1/48 scale. My Williams brass J weighs 14.2 lbs (has a LARGE chunk on lead inside the boiler), that's 75% over weight
My J needs to go on a diet