I haven't been at self publishing for super long, but after more than a year and a half. Book rank seems to only really matter once you start to hit the top 100 lists. Depending on the genera, this can start happening around the 10,000 mark on overall ranking. If I recall correctly. I hit number 1 in sci-fi adventure but was still around 700-1,200 in the overall store. I've only broke that 1,000 mark once on the paid side.
In other words, all books that are not in the top 100 lists are going to be hard for customers to find through casual browsing, unless they are looking specifically for the book. Few people are going to scroll through 30+ pages of books before they find yours. The difference between a book ranked 50,000 and one ranked 250,000 is about four sales over the course of two or three days. Ranking of 30,000 or better is about where one book a day kicks in at. Of course all these rank numbers can swing wildly depending on the season. Spending months like Dec it will take a lot more compared to July when ten books sold will put you on the charts.