For me here in Arizona, I start with 2/3 gallons over from where I want to end up. This accounts for boil off, loss of wort to trub, etc. But the more you brew the more you'll learn your system.
If you end up with less than the intended 5g you can add some purified water but it will lower your OG therefore lowering your final ABV. How much depends on how much you add. Personally though, I rarely check my numbers anymore so I wouldn't worry about it. Just brew and enjoy the experience!
IT Guys Vacuum 2013: Never Forget - WildGingerBrewing
I only like three kinds of beer: lkgged, bottled, or canned - bad coffee
I have some pent-up banning I need to administer - Reelale
Get off my lawn! - shecky
Shut up, old man!- Sharona