Thanks for the chart.
The pattern doesn't give a size in mm, all it says is "bone crochet hook No. 3 or 4". So would that most likely be a UK 3 or 4 (6.5 or 6.0 mm) or a US 3 or 4 (3.25 or 3.5 mm)?
I'm a total beginner, so I have no idea what would be typical for an afghan with worsted weight yarn.