I am surprised DoD kept his points so simple. There is a real science to tongue weight. Here's how I saw it done by an expert:
Right after we got the CD25 on the new trailer, I towed it up to Les at EQ with a Jeep Cherokee. No problems, even though the boat outweighed the rig by better than a ton. When I went to pick it back up, I had the new 1-ton PU. When we hooked the trailer up, Les says, "the back of the truck didn't go down any". We moved the winch stand about a foot forward on the tongue, then he got a floor jack on wheels, a chunk of 4x6 to go between the jack and the boat, lifted the stern a bit, and cranked the boat up to the bow roller. The back of the truck settled down about half an inch, and Les says, "That looks good". Haven't ever changed it and have never had any towing trouble.
As for weight distribution on tandem axles, I don't believe it is as critical as one might think. As the springs (torsion bars, whatever) go down on the heavy axle, more of the load is moved onto the other. Within reason, of course. The biggest factor is the correct hitch height so the trailer/rig is eyeball level, as Dan said. The best indicator of even loading is hub temperature after a long haul on the freeway. Noticeable difference to the touch front/rear for length and side/side for center loading will tell you how the boat is riding. If they all feel about the same and the trailer tows good, then the science is mastered.
Disclaimer: The paper mill where I do my millwrightin' doesn't have too many boat trailers for me to work on. They do seem to have an abundance of engineers, however. Furthermore, when doing the touch test for temperature, don't bother checking the brake discs. They be plenty warm.
