I think that approach, a few bold strokes, will always be kind of a trial and error type of thing. Studying the subject very closely often helps, in my experience. Really understanding what you're painting.
Like M Winther said, you don't necessarily need to add a load of detail per se, but if that is what you intend to do, even if it's just in rough strokes, then I think that it will simply come down to a combination of lots of practice and lots of studying.
Your piece looks really great already, by the way. That likeness is absolutely there. The only thing that kind of stands out to me, is that the hair seems a little bit flat. You did an amazing job on the plasticity of the face, it really has a three-dimensional feel to it, but it does not seem to fully extend to the hair.
However, it's not something that really bothers either, just to be clear. More something that, if you want to, you could look into in a future painting.