Scents
When I first started soaping, I experienced some problems with "vanishing" FOs. The Scent Review Board is a start, but a lot is trial and error. I would never go above 1oz per lb of oils. The likelihood of causing skin irritation is very high, and if 1oz per lb (max) won't do it, then I need to find another FO, cuz they cost too much. It's just not cost effective —as a hobby or business.
I've had people rave over an FO at SRB. and I tried it and it bombed. So much affects how a scent will perform — the oil combo, temps, additives, etc. Best thing to do is buy the smallest bottle available. I don't always take my own advice, but it's still good advice.
Now, I have much more success with FOs. I buy most of mine from Southern Soapers and The Scent Works, with a few from WSP. I've had the most success with Southern Soapers FOs. I'm sure others will have different experiences, based on what and how they soap. Buy from reputable companies, in small amounts and test them.