Simple formula:
- Use test strips for determine the minimum time for maximum black. Then you have a real black in your image. Your highlights may still be muddy.
- Increase paper contrast until the highlights look fine. The minimum time fpr maximum black remains the same, if you don't change contrast too far. Now you have a real black and white highlights.
- Make some minor changings in the exposure time, look more at the highligts. You get even better higlights then.