I failed to understand the need to do any calculation.

Sure, when the bootloader such as grub4dos boots up, it needs memory. But when linux kernel takes over, it reclaims all the memory used by any other previous program(s). As a matter of fact as proven on my notebook when I booted using grub4dos, I still have full access to 4G of memory on my 32bit kernel. I don't have more than 4G of memory, otherwise I could even prove that I have access to 64G of memory on my 32bit kernel.