The ration of powdered beetroot to beets is 1 tablespoon of powder = about three medium beets. I suspect the juice is similarly concentrated. The usual athletic dose of the juice is 250-500 ml. From the performance aspect, the active ingredient in beets is inorganic nitrate. Therefore the simplest and by far the cheapest (it's so cheap as to be almost free) is to take 1g of sodium nitrate every morning. The best athletic studies in fact used sodium nitrate because who knows how much nitrate is in any of the vegetable derived products? The main noticeable effect is to lower blood pressure by a few percent. Studies show it improves performance also by a few percent, no huge thing. Studies also show it has little performance effect on elites, probably because they've got everything tuned to the max. However for us geezers who might not have the world's best arteries anymore, it might be worth trying.
When this data first came out, there were a few racers on BF who thought it made a difference indeed. I've been taking the sodium nitrate every morning for maybe 15 years. I have no idea about the performance aspect, but the BP thing is real and lower BP is always a good thing.
This meta-study has a good overview IMO:
https://www.mdpi.com/2072-6643/13/9/3183/htm