You need to get specific fertilizers designed for cannabis cultivation and not general gardening fertilizers, which might be great for your Grannys' garden, but not designed for cannabis obviously, which has specific needs and these cannabis specific needs are not found in general gardening fertilizers.
You are correct though, when flowering cannabis needs a good supply of both phosphorous and potassium and only a very small need for nitrogen.
Just click on the "nutrients" link on this page, on the left side menu, and a whole world of info will appear.
Hope this helps, Organoman.