A 1.0 would be fantastic. Anything above that is gravy.
Arrow and Legends have been remarkably consistent. Supergirl is all over the map, probably due to Monday Night Football. Legends has held up pretty well against Thursday Night Football, but hopefully the Flash lead in gives it a little more juice.
Arrow's ratings are just a formality at this point. It's going a minimum of seven seasons, probably more. But it's still at well above network average, tied with Supernatural for 3rd overall.
Supergirl is the solid #2 right now.
It'll be interesting to see how all four shows react to the crossover.
Flash generally doesn't bump up much if at all in the crossover episodes, historically.
Generally, episodes with Flash appearing have been the highest rated in Arrow's history.
Flash bumped Supergirl significantly last year but a lot of that was probably just younger CW viewers jumping to CBS; it figures that Flash and Supergirl share a lot more of their audiences than they did last year.
Right now CW looks like this
Flash
Supergirl
Arrow and Supernatural
Legends of Tomorrow
Vampire Diaries and Jane the Virgin
Frequency and No Tomorrow
Crazy ex-Girlfriend
EDIT:
credit for the graph to poster Boon on the spottedratings.com comments