Paging--What happens when page tables do not fit perfectly in a page in Multilevel Paging?

In a 3-level paging system, when every page size is 512 bytes and an entry size is 4 bytes, we can know that every single page has 128 entries. So we can design 7 bits (log128) of the VPN(Virtual Page Number) as an index in the third level page table, which makes the page table fit perfectly in a page. What happens if we just design 6 bits as an index? I think on third level page table, some pages could not be correctly "transfer" to corresponding physical frame number because their page number can not be represented by a six-digit index. For example, when the OS "cut" the whole page table into 3 parts, each of which has 128 entries, and one entry happens to be the 100th entry of second part. How can we use 6 digits to index into 100 to get the right frame number in physical address? Or the "cut" process is smart enough to put just 64 entries and waste another 64 entries in every page? Or we actually can use 8 digits to index into 128 entries table, using just 7 digits and ignore one digit? This problem really confuses me a lot because even if the third level page table perfectly fits into one page, the first and the second level page table sometimes do not fit perfectly at all.