A late addition is reserving most of your extract (75%) until the last 10-15 minutes of the boil. This will do two things - it will prevent your boiling wort's gravity from being too high. Too high means your hops won't be utilized correctly. You can also add more hops to the boil to overcome this, but a late addition is waaay easier and cheaper. It will also prevent your beer from being too dark. In your most recent for instance this is moot...it's a porter. In any case, just make sure you remove the pot from the heat before adding the extract and make sure it's dissolved before returning it to the heat to prevent scorching/caramelizing. Don't worry if you don't reach full boil again before time is up, it is hot enough to sanitize.
If doing the late extract addition you can boil in as little as 1.5 - 2 gallons. More water is better, but that's my personal guideline.
LH milk stout is a fav of mine. Let us know how it turns out, man!