If your important highlights print as you intended on the paper grade you intended your development time for that particular scene is correct. If highlights are too gray development time is too short. If highlights are so light in tone that little or no detail shows development time is too long. With practice you can use your spot meter to determine the spread between important shadow detail and highlights in order to tailor development. Otherwise, you are resigned to pick a development time that allows the majority of frames to print satisfactorily by changing paper contrast and or with dodging and burning. I would recommend against arbitrarily choosing to expose Pan F at 25. Expose at the E.I. that produces the shadow detail you want. Exposing Pan F at 25 may not be over exposing the film. It may be exposing it just right. Developing for times shorter than those recommended by Ilford may not be under developing at all. The shorter times may be just right for your conditions.