2-4oz should give you plenty of foam, just remember to fill the rest of the bottle with water and then shake it up real nice. The PA dial setting should be set all the way down to the PA- setting. At first I thought it was PA+ for soap, but it's actually PA-. Finally, adjust the front nozzle. The default setting is to spray in a sharp stream but you have to twist the spray nozzle about 90 degrees until it's spraying in a large fan spray. If I remember correctly I don't open the nozzle all the way because then the fan becomes too large but I do have it set almost completely open.
While DP Xtreme Foam will deliver the best foam, you should be able to produce amazingly nice foam with any good foaming car wash soap such as Meguiar's Gold Class.