There's lots of discussion on every forum about boot sizing that's why many don't really want to answer you.
But I'll try to help... I just bought a pair of Nike Kaiju boots this summer too btw.
First of all, your regular shoe size means nothing. People have different preferences when they wear their shoes, some like it snug fit, some like to wear 1 or even 2 sizes bigger. Usually people will say "oh buy 1 size or 0.5 size smaller", but that's just because on average people wear 1 size bigger. That does not mean that you are actually doing that with your "regular shoes" though.
So what you gotta actually figure out is your actual foot size. The best way to do this is just to go to a store and try out boots, or ask them to figure out your size for you.
Once you got that, then you need to be looking for a pair of boots that's as tight as possible on your feet without hurting them. That's, again, USUALLY half a size smaller than your actual size. If you are gonna buy online, you still might as well try sizes in a local shop before you pull the trigger.
Also worth noting specific to the Kaiju boots is that someone (that seem to know what he's talking about) mentioned on a different forum that the difference in half sizes for the Kaiju boots is just the liner. For example, size 8.5 = size 9 boot, size 8 liner. So you might want to keep that in mind when people mention that their boots feel "narrower" and stuff.