Storage space is running out - some system functions may not work topic

I have the 32GB N5, and currently have about 500MB free. That seems like it should be enough to me, but Android obviously thinks differently. I called Google Nexus support and talked to someone who had zero knowledge/training, so no help there, they just told me to delete stuff until the message disappeared(which I find to be an insulting "solution").

My question is, is this message a bug? And/or what is the minimum amount of space that needs to be left unused for all of the "system functions" to work correctly.

My appreciation to anyone who can answer this question.