If we agree that the pools are the biggest and the softest, then there are not ever any better opportunities, never mind everyday (for horse races, in North America).
When you say \"too many variables\" what you\'re basically describing is variance, which is a good reason to play within your bankroll, but not a good reason to take anything less than the biggest swing you/your bankroll can afford to take.
Also, remember that as your EV increases, your risk of ruin decreases. In other words, the more that you have the best of it, the more you can/should bet.