How can I assign an amount of XP points to an enemy that I have created for a campaign that I am writing? What would be a good way to decide, and still be fair? The campaign that I wrote is for the Fallout PnP, specifically.
One way is to use the Fallout PnP Bestiary and compare the existing creatures. Find the two creatures with the most similar stats to the one you've created. Pick one that is slightly higher and one slightly lower and then give your creation an XP value somewhere between the two.
For instance, if you created a medium radscorpion with the same attacks as a Lesser but the armor of a greater, you would pick the XP value halfway between the two (300).
The D&D way to do it is to decide how many kills it should take to boost you up a level (13 in D&D 4E), then find the level that you are expected to have an even shot at killing the creature (for this example, say 12) and divide the required advancement experience by the expected number of kills (11,000/13 = 845).