Page: A fixed length of contiguous block of virtual memory.
Segment: A segment is your interface into the shared memory. A segment is made up of one or more pages. If you (or your process) haven't created a segment, you're not using shared memory.
ceil: AKA 'ceiling'. A well-defined math function that returns the next highest integer (aka rounding up). See Wikipedia: Floor and ceiling functions
PAGE_SIZE is the number of bytes the OS is using to split up its' chunks of memory. You can find the size with getconf:
# getconf PAGE_SIZE
shmmax is the maximum size of any individual segment in bytes (not pages).
shmall needs to be at least 'ceil(shmmax/PAGE_SIZE)' because if it was any less you could not create a segment that's shmmax in size. You would run out of pages to use.
Let's say you want to use no more than 8MiB (MB being base 10, MiB - mebibytes being base 2, what your computer actually uses when calculating the sizes) for shared memory on the system.
To find the number of pages 8MiB is you simply divide by PAGE_SIZE.
8MiB / 4096 bytes = 2048.
Set shmall to 2048.
Now let's say that you know you only need a single segment that's 512K (KiB, not KB) in size for postgres. You have all the data to calculate the minimum number you should set shmall to.
ceil(512KiB / 4096 bytes) = 128
The smallest you should set shmall to would be 128 pages. There's nothing stopping you from setting it higher. Shmall is simply a limit specifying that you will not use more than that amount of memory for shared memory whether there's one segment or ten.