Yes, that ratio will help you determine about what NSS you're at, and I actually did a running total for myself just to see where I was (already discussed elsewhere, but I think not knowing ABOUT where you are, at least compared to other studs, sucks). As mentioned, however, NSS changes month to month, so you'll never be exact.
In VT-2, the 50 break was around 1.139 or so a few months ago, 1.141 in June, and in July a 1.154 = 55 NSS (don't know the exact 50 break). It changes from squadron to squadron too. This is also assuming everyone had the same total graded items, which (if you look at the MPTS equation) has some weight, as well as your academics in Primary and API. So it's not a failsafe way, but should be relatively close.
If you really care near selection time (since RI's will bring it down a lot), calculate your ratio and compare it with guys who selected during previous weeks of the same month from your squadron. That should tell you ABOUT where you are.