Calculating correct tongue loading for trailering a Roadster

https://www.etrailer.com/faq-h...r-tongue-weight.aspx

Found this link

The technique makes sense, but what is the tow vehicle?  Different tow vehicles work best with different tongue weights ( the tongue weight allowable is either on a door jamb sticker or in the owner’s manual).  I had to shift my speedster forward 4” on the trailer for a stiffer sprung F250 versus running it back farther for an F150.  Too far back and the trailer would wallow left and right above 35mph.  Too far forward and the truck’s rear end hung too low.  Happy medium gave a 3” drop on the rear bumper when loaded and no wallow. 

Percentage to tongue weight is just a ball park recommendation...try this : With the trailer hitched to the tow vehicle and not loaded  measure the tongue height. "Load the car on the trailer positioning it so the tongue is 1-1/2"  lower than first measured, secure and go for a ride and see how it feels , reposition forward or aft until you are satisfied with the steering and braking then mark that position on the trailer deck. Be sure to take into consideration extra cargo and passenger weight in the tow vehicle

2018 Dodge ram 1500 quad cab sport Hemi w/  8 speed trans. . Allowable tongue weight is about 1,100 lbs. 

Earlier we towed a 5,500 travel trailer with a similar truck with no concern for tongue weight . 

Wow. I’ve never considered weighing the tongue.

I just load as much as possible over the trailer axle. If the trailer wags at speed, I move the load forward to get some tongue weight. I want the least amount of tongue weight that works (you’ve gotta have some). For a 3000 lb total load, that means I can lift it and push it over to the ball and such by hand. 

Yup.  That’s how I roll, too.

You are over thinking this.

The trailer is so light and so is the car that you will undoubtedly be within any maximums. But given your weight estimates, the 10% rule should be applied. Your 3500# weight should have roughly a 350 lb tongue weight. A homemade(?) trailer may have a bad balance vs a production made trailer. Better to be tongue heavy than tail heavy (makes for tail wag or sway). Also measure your truck sag before & after- it shouldn't sag excessively.

Hopefully you are using a tandem axle trailer... Balance your lightweight car with the motor/transaxle just behind the axle (s) and the rest forward. Do as Gordon says and adjust so your trailer does not wag back and forth. More importantly is having electric brakes for your weight, AND, having a steady & smooth driving style. Any trailer can be pulled poorly.

My current setup, balanced as described... (apprx 5K combined)

As Stan says get a little tongue weight. Trailers like that. 

Don't ask me about the time my 20-year-old switched my receiver from the 2 5/16 ball down to the 2". Hooked up to the enclosed car trailer (full) in the dark. Didn't notice until it came off at 65 mph. Quite an experience. Seeing the damn thing fish tailing behind the F150 -up on two wheels one side then the other. Kinda wished I had a piece of coal in my backside -would've had a diamond by the time I came to a full stop.