There isn't a single set distance between the two VP's needed in order to get 90 degrees. The distance will depend on the type of lens effect you want to produce.

Here is a video timestamped to the relevant parts about lenses

https://youtu.be/2XF5YuAK63I?t=251

In short, if you want a dramatic, distorted, look then bring the two VP's closer together. This emulates a wide-angle camera lens.

If you want to get closer to an isographic, more regular, look then you spread them further apart. That emulates a long-angle camera lens.

So, with all that said, that doesn't really help you get a good box like you were asking for. I recommend starting off practicing what is called the Y-Method. IIRC Uncomfortable mentions it in the box challenge material. In any case, the Y-method has you draw the closest corner/edges first and.

The discord has this image pinned in their Lesson 1 channel that might help:

https://cdn.discordapp.com/attachments/368870697742630912/384092950700752896/BoxDemo.png

Notice that the image states the angles between the lines should never be less than 90 when using the Y-method. (You'll also see what is called The Arrow Method for when you have a 2-point-perspective box that is placed directly in front of the horizon line. I'd say stick with the Y-method until you get the hang of it.)

I feel this video from Proko does a good job breaking down the intuitions you are wanting to develop as you draw a box using the Y-method:

https://youtu.be/3uEtdDvK6Xo?t=308

It isn't timestamped exactly to the Y-method part but it is worth watching the entire segment, in my opinion, so I timestamped it there.