Have always had smaller (16' and smaller) boats. 10% +/- tongue weight always worked fine.
Going to a 20' with tandem trailer, total weight for boat/motor/trailer/gear, 3700-4000lbs.
Do you still figure around 10% tongue weight with tandem axles (400lbs sounds heavy)? Are tandems any more/less sensitive to tongue weight? I assume they are more sensitive to tongue height.