Final Volume

Hi all...I was wondering how the majority of you all account for the amount of wort left behind in the BK. For example: if I lose 1.5 gals during the boil and leave another 1/2 gal behind would you start with 2 gals more than what you would want for a final volume? This may sound silly, but I've been having problems ending up with the correct volume and I'd like to know if there's something I'm missing. Thanks

given your statements...you should start pre boil with 2 gallons more than your target final volume