metalness workflow is different, but pbr doesn't mean you can not use the specular workflow. you can still use physical based values going the specular route.
using a traditional specular workflow you'd plug the spec map into your specular color channel inside of maya. Spec maps should always be created from scratch, inverting your diffuse will only produce results out of sheer luck. at the most basic level white = full spec, black = no spec, though a pure grayscale spec map is…