Typically low awards are available only 331 and 330 days at. At 329 days out they almost always become mid.

That is what I've observed. The two green days are always 330 and 331 days out. What was low yesterday (330 then) is mid today (329 today).

This is a common pattern that I see on many flights.

So, typically need to first book one way first, then book return later - to catch it at 330 or 331 days out, before it's day 329 and it goes up to mid.
(That is, if it's a route that even has these two days of lows...not all do. Many are always, no matter 331 or 1 days out, high with a few mids thrown in, even though flight may be more than 1/2 empty flying out.)