I did the same thing with my enclosed car trailer. I don't know if there is a formula to figure the ratio, but I have a tongue scale that I use.
I welded on a receiver at the back of the trailer(50lbs). Then added a small motorcycle carrier that weighed 50 lbs. I then loaded a 300 lb. scooter onto the carrier for a total 400 lbs of extra weight added to the back of the trailer.
Without the carrier or the scooter on the back of the trailer and my car and atv loaded in the trailer, my tongue weight was 1150 lbs. With the carrier and the scooter loaded on the back, the tongue weight was 1075. Adding 400 lbs of weight on the back only took off 75 lbs of tongue weight.
I would suggest getting a tongue scale(about $50)because the tongue weight could vary a little, or a lot, depending on where your axles are positioned on the trailer.
