I could be mistaken, but I think that the best way to go about determining what size crochet hook to use with a given knitting needle size would be to measure the size of both the knitting needle and the crochet hook in millimeters rather than in US sizes.
For example, a US 8 size knitting needle can also be found labeled as 5mm. I would then find a 5mm crochet hook, which would be a size US H.
You can find conversion charts for both knitting needles and crochet hooks for converting between US sizes and millimeters.
Hope this helps! I'm sure others who are experienced crocheters will have insights for you as well.