Clublength for junior set of clubs

[cosine]

I'm going to be building a set of clubs for my younger brother, but can't seem to figure out the proper shaft lengths for his wrist-to-floor measurement. I'm going to be building him a 3-wood, a 3-iron, a 5-iron, and a 7-iron (all components bought extremely cheap during Golfsmith's FF sale ). His wrist-to-floor measurement is 28 inches.

Thanks for any help.