I think it's better to compare House popular vote with the average margin between candidates across all Congressional Districts.
I found this from 538 back in 2010:
As Nate Silver put it, "While the generic ballot has tended to overestimate the Democrats’ aggregate popular vote, it has not tended to overestimate their average vote in individual Congressional districts — or at least not to an extent worth worrying about (and Gallup’s poll has tended to underestimate it). Since our model operates at the level of individual districts, this convenient property allows us to take the generic ballot at face value: if it shows the Republicans ahead by 7 points, the model takes this as evidence that they will win the average Congressional district by exactly 7 points."
Here's the catch though: If Democrats do overperform this year, it may not necessarily be to their benefit, either. It does no good to run up the margins in safe districts while losing close races.
Here is the link:
https://fivethirtyeight.blogs.nytimes.com/2010/09/24/the-bias-of-the-generic-ballot-its-complicated/?_r=0
