Stereo & Electronics - Fuse box power stud size

I'm installing some HIDs (and a small amp) and I went to connect to the power stud on the side of the fuse box for my relay harness connections. But the ring terminal won't fit over the stud! Dang it. I don't have any calipers so I broke out the ruler :lol: It looks like the ring terminal I have is something like 5/16? I can't accurately measure that power stud with a ruler.

What size ring terminal do I need to fit over that power stud? Is 3/8" the size I need or do I need 1/2"?

Last time I put HIDs on my old car I connected straight to the factory harness. I suppose I could still do that as I had no issues doing it that way, but since I have the relay harnesses I thought I might as well do it the "right" way. But now this put a wrench in the works.

Just come straight off the battery, that posts cable already has to handle the current for every thing in the car. and GM quite frankly undersized these cables from the get go.

I'm coming off the post because the headlights already do come from the fuse box, I'm just rerouting through the relay harness. Actually they are going to draw less power than the factory lights did (35W each vs 55W each) so if anthing I'd be reducing the load on that main wire by a little bit. Coming off the battery is more of a pain with the dumb side mount terminals, which is why I want to just use the fuse box stud.