Over the weekend British Airways had non-stop flights from Seattle to London for $660 RT for the month of August 2018. They are still a fairly good price today at $781 RT. These are good prices from the West Coast for Non-Stop flights in the summer. Dublin, with a connection at Heathrow, $800. (My sister and I bought the Dublin flights) I feel that these RT prices are probably about as low as a summer flight will go and August is the only month we could travel.
I subscribe to the theory that you should have an idea of typical airfare prices and buy when you feel comfortable to have had a reasonable deal. I am doubtful if I waited until March to buy these tickets that I would get a lower price than this. I also like "spreading" my costs out a little. The pain of buying these tickets is over and I can start saving up for lodging next.