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I am assuming that you are doing a mash in a bag with your cooler (or similar) mash tun. In this case, you want to add your 0.5 gal to your mash tun dead space. This will account for the half gallon you leave behind. I am guessing that you will perform your mash and then drain just the amount you need to meet your pre-boil volume into your kettle.
If you are doing a more traditional BIAB, i.e. mashing and boiling in the same vessel, then you want to add your half gallon to the trub loss at the end of the process.