I'm not sure how much of a good deal you'd get going to the US if you booked last minute. Lots of people travel there for business so they tend to hike the prices up close to the time for that reason. It's also a lot more expensive if you're not there over a weekend as they assume it's a business trip.
I fly short-haul semi regularly and have found that 4 months in advance is a good price (may be similar for the US but not sure!) My advice would be to sign up to sky scanner/kayak and keep an eye on the prices there for a few weeks/months and you'll get an idea of how it fluctuates. If you're going in Easter though you have loads of time.
Also always search + book flights in a private browser and direct from the airline. So use sky scanner to figure out the prices and then go direct. Your cookies will tell them that you're looking to fly on certain dates and they'll hike prices up to encourage you to panic buy... which could explain the 450 increase.