Details about the options for pressure outlet to be found in the Fluent User's Guide. If not using profile the profile multiplier is not used at all.
You should place the outlet at least 10-20 times the diameter of our pipe. The best place is really where you can provide reasonable backflow stagnation quantities if reversa flow is expected to occur or best where no reversal flow is expected. I would say free stream gauge pressure is the way to go.
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