I'm trying to fill in this corner to be precisely square (**see the image below**). There are several ways I've come up with, but they're all terribly time-consuming and inefficient. I'm new to Blender, and I'm 100% sure there is a better way. I'll save us some time and not explain the methods I came up with.

How would you fill it in?

[![Enter image description here](https://i.stack.imgur.com/cvoLY.png)](https://i.stack.imgur.com/cvoLY.png)

The red lines I added to the image is in paint as to visually represent what I'm asking.