Tirebiter, that's a really fine trailer! Well done.
Rule of thumb for utility trailers is that the tongue weight should be 10% to 15% of the total (gross) weight of the trailer (excluding cargo).
Empty 400 pound trailer = 40-60# tongue weight.
It will take trial and error to get the chock in the proper place.
Edit: my trailer toting neighbor informed me the tongue weight is calculated without the cargo.