I apologize for the use of the word "legally". It is an interpretation of the FAA and a guideline of the CBO. I refer you to he FAA's guidelines for Model Aircraft at https://www.faa.gov/uas/model_aircraft/ and the AMA (and others) "Know Before You Fly" at http://knowbeforeyoufly.org/for-recreational-users/

I have not read the NPRM, but I would be surprised if that limit (or the 500' limit used for commercial UAS) does not come up.

Then of course there is the FAA's "AC 91-57, Model Aircraft Operation Standards", (which may or may not have been rescinded), which says:
Do not fly model aircraft higher than 400 feet above the surface.