Target Volume for OG estimation

[MillmoorRon]

Just planning my first brew after satisfying myself that I've now got all the gear I require, though there is still one thing I'm not clear about!

I've been using Beer Engine as a way of seeing the effect of different ingredients on colour, bitterness and expected ABV, but what exactly is the target volume?

Is it:
Volume of wort collected after mashing?
Volume of wort after collected in the fermenter after boiling?
Other?

For small batches, boil losses and dead space in each vessel could be quite significant!

Confused!