I'm not an expert, but some of these recommendations sound like too
little weight on the trailer tongue to me. You don't want to drive
over some bumps and dips and have a lot of upward pressure on your
trailer hitch ball. If you take a look at the hitch mechanism you'll
see that it's not made to withstand a lot of lifting (upward)
pressure. It's designed to withstand a load from above. If you've
ever forgotten to release the ball before you used your tongue jack on
a boat trailer, you know how easy it is to pop the trailer tongue off
the ball.

Also, my understanding is that the weight of the trailer is supposed
to be included in your calculations.

A suggestion: Call a boat manufacturer (or a travel trailer
manufacturer) who sells a rig with dual axles, and of approximately
the same over all weight and length as the rig you're putting
together. Ask them what the tongue weight is on their trailer. I'm
not saying to copy that. But it may give you a practical place to
start.

GL