Hook size confusion

[mrducky]

Just when I thought I had knitting needle size designations figured out, I get even more confused by crochet hooks!

I'm starting an afghan, from a sea-shell pattern I found. It says use worsted wool and "bone crochet hook No. 3 or 4". I found this chart, so I'm guessing it is the U.K. designation which means 6.5 or 6.0 mm. Does that sound right?

It looks reasonable to me. I'm not too concerned about the gauge, but I don't want it to be way off.

Thanks for any help!