So just to clarify what (I think) I know:
PL is promoted listing. You select a rate and that will be the rate you pay if your items sells.
Dynamic is a floating PL rate. You agree to pay the highest going rate on the day to get visibility. So if your items sells, well, you could be charged 98%! Which is why dynamic has...extras...
(1) You can select a CAP. If you select a cap of say 10%, but you only needed 8.9% to get the visibility you needed to sell, the idea is you will charged 8.9% not 10%.
(2) You can select a MODIFIER. i.e. You can agree to pay a % above or below ebay's suggested amount. The modifier is great for ditching stock (at a premium of course)
Obviously you will never know if ebay has been honest.
I use dynamic. Every day I add all advertising fees (PL, offsite, shop) onto my own spreadsheet. I then divide that figure across ALL sold items. So my PL fees for this year ytd are 6.09% as a % of total sales with £4.93 per item sold. Obviously on an item I sell for £10 (which is very rare) I wouldn't be happy. But given that many of my items sell for well over £100, it feels manageable. I take this overview on advertising as a % of sales because it is hard to know synergies. People may look because of PL. Add me to their fav sellers list. And then I get a sale months later through a coupon email offer etc.
I hope this helps
