How many times have you loaded a trailer and guessed the tongue weight then went for a ride and felt the trailer start "wagging the dog". Oops, not enough tongue weight. This is a very dangerous way to do it. Up until recently the only way to get the weight balance right was to disconnect the trailer and use an expensive comercial scale under it to check. That was the past.
Weigh Safe hitches have a built in scale to show you the tongue weight. No guessing.
Tongue weight should be no less than 10 percent and no more than 15 percent of gross trailer weight (GTW).