Interpreting sys.dm_os_sys_memory

I try to figure out a way(using only TSQL) to determine if and how much the PAGEFILE (I will capitalize this term if I refer to the windows page file from now on) is used (like pagefileusage% counter).

My first thought is using sys.dm_os_sys_memory. I am little bit confused here. The description of the total_page_file_kb is that this is a commit limit so basically = size of the physical memory (RAM) + size of the PAGEFILE So far it is ok. But what is exactly the available_page_file_kb? Is it referring to the commit limit or to the PAGEFILE? I tried to use these calculations:

Assuming that available_page_file_kb referring to the commit limit

PAGEFILE usage% = (available_page_file_kb-available_physical_memory_kb) / (total_page_file_kb-total_physical_memory_kb)

Assuming that available_page_file_kb is referring to the PAGEFILE

PAGEFILE usage% = (available_page_file_kb) / (total_page_file_kb-total_physical_memory_kb)

Unfortunately none of these calculations show me the right results. I keep checking the pagefieusage% counter on some of my test servers, and sometimes the first formula is wrong by only 1-3% an the second one by 15-25% or vice versa, the second one by 1-3% and the first one by 15-25%

What did I miss here? Is it impossible to determine (or even just get a close estimate) the pagefileusage% with sys.dm_os_sys_memory dmv?