I've always wondered if the BB calculator was based on total batch weight.
The way it was written in the calculator, I guessed that it included the water, lye and other additives. I'll give you a reason why I think that's actually better (in some cases)
When we look at maximum fragrance usage rates, we're looking at the maximum dilution in the whole batch. If you're looking only at the oils, you're missing about 30% of your total batch weight and your calculated maximum amount based on oil % would actually be lower than the true maximum.
I was reading a manufacturing book on soap once and I believe the soaps are pressed (milled) until they have a moisture content of less than 15%.
Sometimes, when usage rates are low, I will incorporate all ingredients (lye, oils, water, salt if applicable!) into a batch weight to see what the actual usage rate is. I also calculate my water as being 12% of my batch weight (I don't remember where I got this number but its what I use).
This method is very useful for salt bars or other soaps with high amounts of additives. You don't want to just use your straight full water amount as you know you loose a lot of water during curing. But, if you're using less than 15% of your total batch weight as water I would just include it in as you aren't going to get too much more water loss at that point. You can also always just add the lye, oils and other non-volitile additives without including the water if you're worried about it.
Is it easier just to use PPO/%O? Yes, and its also safer in some ways as it has a built in buffer. However, if you have a program that already does it (like soapmaker) or want to calculate it out, it is technically more correct to use the % of total batch if you want to get the most out of you FO/EO.
I did this recently with Anise EO which has a 0.9% usage rate. I used 0.8% of my total batch which ended up actually being 1% of my oil amount. Is it probably significant? No, but it makes me feel better that I have a record that I'm within safe usage rates. Thats my take on it at least!
ETA:
In reference to the actual sale, it looks like I'll be actually participating even though I originally didn't want to because my "restock" of an FO from a different supplier smells nothing like the 3 other suppliers I've tried of it! It's Lavender Chamomile. It's the same as Bedtime Bath from WSP and Lavender Chamomile from Mad Oils but not the same as Bedtime Baby from NG.... Grrr.
