There's no real formula, other than buy at the price you are happy with.
I bought a R/T ticket for my Sep trip back in April, when the pricing was at $368. At the time, guys I work with who travel regularly were paying over $500 for R/T tickets to FL so I perceived that as a great deal.
I did upgrade the ticket to Main Cabin Extra to choose preferred seating without any fees, get a free checked bag, and free drinks (plus no change fees if I needed to make any switches). That pushed the ticket another $100 but for me, it was worth it (figure $60 to check a bag R/T, and another $50 for seat selection fees in that front section).
Prices did come down...the itinerary is now at $326 for the cheapest ticket. I ended up needing to change my return flight to a later one (yay! More time at Disney!) so no cost to change flights...BUT the best part? The First Class fare came way, way down...and I was able to upgrade that MCE fare to First Class for just $50 R/T.