Sparge to 20 litres.
Boil about 5 litres for 60 min.
Heat wort (separate from boil) to 165 - 170 F.
Put wort into fermenter to cool while the other boils.
Cool boiled wort, strain hops out and decant into fermenter.
Add cooled boiled water to fermenter if necessary to make 18 litres.
If you end up with 19 litres, just ferment 19 litres; it'll still be good.
An interesting variation would be to substitute 5g of 5-min Southern Passion for 5-10g Vic Secret (also at 5min). This would yield more passion fruit flavour. I find it a bit rich since I brew to drink myself, and drinking one of these every other day gets quite much (this is about 2 months worth of beer for me). As a sharing beer, it's really excellent.