Please put a magnifier on the 4 points when manually applying keystone correction using the guide.
It is very tedious to enlarge the image and then move the points around. If the points were automatically magnified, it would definitely speed up the process.
The Nik 6 Perspective in the Nik collection has this feature and is very useful, but I prefer to work with the CAPTURE ONE.
Like 370131390338 I had been asking for brining in existing Loupe mechanism into Keystone and Straiten tools many many times. In fact during one of the Keystone update they added. During Beta testing I found it broke the actual Loupe tool functionality and submitted as bug. Instead of fixing it they removed it. I thought would be be released in one of the subsequent releases but it didn't happen.. Sad....
This has been requested a couple of times already. See these:
Keystone tool: Loupe showing position of adjustment points
Add loupe to keystone tool