Efficiency numbers are highly dependent on accurate volume measurment and proper accounting for all losses wherever they may occur. So, what I'm getting at is that when calculating the efficiency in the fermenter, you would take a gravity reading and a volume measurement. There are sure to be significant losses along the way to the fermenter. Some wort will be left behind in the mash tun, some will be absorbed by the hops and some may be lost to hoses or chillers. The best way to get a handle on the efficiency is to measure it pre-boil only. This eliminates the hop absorption issue and there is likely little sugar remaining in any wort left behind in the mash tun. You still need to get an accurate measurement on actual volume in the kettle, but otherwise it's fairly straightforward. As for the fermenter, you will want to take a starting gravity reading so you know how close you got to your target gravity, but I would not try to calculate my efficiency on those numbers. If you missed by much, just make adjustments next time accordingly.