I assume you already have the Trailer? Use a cheap set of bathroom scales under the tongue jack which has been placed at the level it will be when hooked to the car. Put the bike on the trailer, with rear tire of bike over to slightly behind the trailer axle then Move the bike backwards or forwards to obtain about 10% of the total trailer/bike weight on the tongue. Then mark that location of the bike, unload the bike and install your stops so bike rides there. When we sold trailers this is how we set up all small load (motorcycle/atv etc) trailers.
Trailer weight can be obtained from manufacturer and good ole HD will give you approx bike weight, add your load in saddlebags. Approx will work fine, not critical to a few pounds.