Yeah, not back - forward to lighten the nose
Depending on your tow vehicle.... (a lot depends on that and the weight of the lump hanging off the transom) you want to be able to just about lift it with one hand - 60kg ish is a good starting point. It's all down to you and your set-up though.
Easy way to do it is get a set of bathroom scales, put them down in front of the trailer, under the hitch. Chop a length of 2 x 2 or similar to around the same height as your towball (around 19" ish?...), stick it in the towball socket on the coupling head and the other end on the bathroom scales. Then wind the trailer down, on the jockey wheel, until it's about that height as the towball, and see what the scales read.