Perhaps if you clarified what do you need the FPS for, we could have helped more. I just assumed that you wanted FPS to display or whatever. FPS is Frames Per Second, which is an average. However, if you use fixed numbers like that for your equations, you might notice disrespectancies when your FPS drops or rises.
Timedelta expression is ideal, because it scales with current FPS. When your game gets slowed down due to all those projectiles on the screen, timedelta rises, keeping the game flow uniform. Stuff that uses timedelta runs equally fast on 30 FPS, 60 FPS and 500 FPS.
So, the question is - what do you need the FPS for? What is that you are trying to do?