10:55 PM, Saturday March 27th 2021
I'll try to help you at the best of my current knowledge.
The angles in your initial Y should always be wider than 90 degrees. This one seems a minor thing but I have noticed that it's very important in order to draw a box in three point perspective.
The initial Y alone doesn't determine any Vp. You need at least two parallel lines to establish a Vp in space.
About choosing a Vp: if you opt for a shallow perspective, you'll find that the VP is almost always outside the boundaries of the paper. I might suggest to not stress too much about "finding it". Once you have a set of parallel lines, try to imagine where those lines converge and follow with the remaining ones of the same set.
Even though there is no standard, from all the posts I've read I would say that It's perfectly common that one doesn't notice any major improvement after a few dozen of boxes.
ScyllaStew made this sept by step video guide that you might find useful.
Good luck with the challenge and if you have any question feel free to ask.