Well 12% is pretty damn lean and 8% will be REALLY lean. I'd love to be 12%!!!
I noticed you said you follow PB "most" of the time. The closer you get to lower body fat targets, the harder they will be to attain. To get to your desired level, it will probably take perfection in everything (sleep, food, supplementation, workout frequency/intensity, etc.), and YES, you will most likely need to restrict calories by some measureable amount. And because of the precision that it will require, YES, you will most likely be successful using a weighed and measured approach.
Expect your performance to drop off both as a result of the calorie restriction and the further reduction of body fat...even though you will be at the upper end of "essential" body fat (assuming you're a man), that does not mean it will be optimal?
The real question I guess is "why" do you want to be 8%? What is the significance of that number? Is it simply an arbitrary drive to be at single-digit BF%, or is there some athletic and functional reason why want to be 8% BF?