Sorry, I missed some of the later replies until now ... good info from everyone (thanks Rod). Like I said earlier, I'm running two ads for the same book one in the US market and the other UK. Genre is historical fiction/WW2 with a strong love story ( not romance, really). I have $5.oo daily limit.
ROI is still double my costs, so I'm not touching a thing. Because both ads are neck-and-neck it's tempting to bundle the target audience into one ad and double the budget, but they're working, so...
It's opened my eyes to potential KENP income with 100k pages read per month.
Meanwhile, a third "test" ad for a different book/genre/market is getting clicks but no sales. Makes for an interesting conundrum, because I'm allowed to have belief in my writing given the success of other titles, so it's something else I've got wrong.
Oh, and I agree with answering comments completely. I often wonder about the mantra of not reacting or commenting on reviews. We're always looking for reader engagement, yet reacting to reviews is frowned upon.