I usually track it for using half of the marinade ingredients. So if the marinaded uses 1/2 cup of olive oil, I track it as 1/4. That's probably overestimating, but it's the easiest way.
The other way to do it is measure the entire amount of the marinade before you put the meat in, then remeasure it after you take the meat out. The difference is what actually went into the meat.