An intuitive measure, based upon assessment of training quality, how you "feel" on the day, degree of effort you put in etc? And also progressive, based on racing history?
Any time is a good time if I beat people from my old club. Haha!
If I've done a race where I couldn't have put one more ounce of effort in then I'm happy. Although possibly that mindset is bad because you fight your way home, muscles tie up, and you get slower.