I think the general guideline is between 0.7 to 1 oz FO ppo.
I'm new to soaping too (only started a few month ago), so haven't have it all figured out. However, I do tend to like my stuff more fragrant, so I usually go on the higher % side. I think usage can also vary from scent to scent, depending on how strong your FO is. You kind of have to test things out and see how you like it.... HTH.