You can also determine the tongue weight a different way, and I think it's a far better approach, because you really want to know the weight on each axle. This method uses a CAT scale at a truck stop, but could be adapted to any other kind of scale:
Connect the trailer in your normal towing configuration and put the trailer axle(s) on the rear platform, and each of your tow vehicle's axles on the two front platforms. Get that weight. This tells you the weight on each axle. Add them up and you have the Combined Gross Weight. Your tow vehicle has a limit on this.
Drop your trailer on one of the truck stop's scales so that its axles and tongue
are all on the rear platform. Move your tow vehicle so that each of its axles is on one of the separate front platforms. Get that weight - now you know the total weight of your tow vehicle (the sum of its axle weights), the total weight of the trailer and, the weight on each axle of your tow vehicle.
total trailer weight - trailer axle weight = tongue weight
And, you know how much additional load you are putting on each of your tow vehicle's axles. Ideally, you would like the weight distributing hitch to apply an equal load to each axle, but you'll probably have to settle for something "close". And of course there's a limit on the axle weights for your tow vehicle too, that you won't want to exceed.
It cost me $9.00 per weighing here at the TTT Truck Stop CAT scale in Tucson. Just talk to the folks at the truck stop first so they'll know what you want to do and I'm sure they'll be very helpful. (And, they'll tell you what to do when you're on the scale so you won't just stand there feeling like an idiot - you probably have to push a button that signals them to take the weight and you can talk to them through a loudspeaker too.)