I used to do an approximation.
I was having a pistol that wouldn't run. I wanted to be more precise and track failures. Well, being a computer scientist and a software developer, I wrote an "App for that".
http://gunlogapp.blogspot.com/
Well, the app grew and become more handy so I put it out for those iOS users out there that have an extra buck in their pocket.
So, now I have a very accurate count and I have photos of my targets associated with the weapon and ammo and I don't have to go through my disorganized stack of targets/papers.
So, I started wondering, when was the last time I cleaned my weapons, so I added that as well.
Then I added a report area. Failures can be found easily, round count, etc.
The app tracks just about everything now. It is getting big!
So, that is what I do now. I am more concerned about results and failures, so I often do not store all of the information that the app can store.