We ramp launch a J24 at various regattas several times a year. The ramps are all long enough, so we do not have that worry. What we do, is back the trailer near the water and chock? it with a rock or brick. Then we tie a long rope to the trailer and the truck. The rope is 40 to 50 feet long. Disconnect the trailer from the truck. Pull the truck forward until tensioned and remove the rock. Back up with some speed. ( We have a 13" fixed wheel that does not swivel on a jack, but you can use the normal trailer jack wheel. When we were using the normal jack wheel we had someone run along and try to keep it straight which was not hard.) The trailer can be stopped, when the boat is afloat. We have a loong dock line and pull the boat to the dock. Pull the truck forward and revesre the process to hook the trailer back to the truck.
Tie the boat to the trailer at the bow, or use some type of loop on the keel until you are at the water or the boat may fall off on the ramp.
I cannot post links, but google Schroth boat repair for some pictures. Also go to j24texas.com and click Ramp Rocket at the top of the home page for another bad result