I agree, its confusing for many and that confusion arises from the fact that you usually talk of shared_buffers in MB or GB whereas hugepages have to be configured in units of 2mb. But once they understand they realize its pretty simple.
Don, we have experienced the same not just with postgres but also with oracle. I havent been able to get to the root of it, but what we usually do is, we add another 100-200 pages and that works for us. If the SGA or shared_buffers is high eg 96gb, then we add 250-500 pages. Those few hundred MBs may be wasted (because the moment you configure hugepages, the operating system considers it as used and does not use it any more) but nowadays, servers have 64 or 128 gb RAM easily and wasting that 500mb to 1gb does not hurt really.
I don't have a problem with the math, just wanted to know if it was possible to better estimate what the actual requirements would be at deployment time. My fallback will probably be you did and just pad with an extra 512MB by default.