I need to prepare YPD plates containing hygromycin, but I don't know which concentration I can use. I'll select transformants of Hansenula polymorpha.
According to this site, for yeast (I assume this means Saccharomyces cerevisiae) you should use 50 µg/ml to 200 µg/ml; for fungi (!) use 100 µg/ml to 300 µg/ml. The site also stresses the importance of the pH of the medium.
I think that, unless a Hansenula expert comes along, you will have to try an initial experiment to measure the sensitivity of untransformed cells across the range indicated. Then you can try selecting resistant cells at a concentration somewhat higher than that which kills untransformed cells. Finally, you can check to see how resistant the transformants are, to enable you to decide on a useful concentration for routine selection of transformants.