NEPA Guy. After your trailer is hooked to the truck. Find a good spot you can measure from the back of the truck to the ground. Take a measurement. Then load the tractor on and measure again. You should look for around an inch or two of drop on the back end of your truck. That should get you in the ball park for the correct tongue weight. Course the only way to tell for sure is a test drive to see how it handles.
Once you find the sweet spot, either mark the deck or reference where the tires are to the side rails, or fenders in the case of your trailer.
On my 16ft dove tail, the sweet spot for loading my tractor is to back it on. And park the front tires right on the edge of the slant where the tail starts to drop. Course I don't have a fel, or I would have to change how I load it. Especially with implements.
You'll have some trial and error till you figure out what's best for your set up and attachments. And trust me, we can all give you advise, but every one of us has a different truck, trailer, set up, and implements. And how we load, and position on the trailer changes based on attachments attached to the tractor, or whether we have to haul multiple attachments