G'day Paul,
your not doing anything wrong!
your maths are good, it was mine that were flawed, I was only adding gig width to diameter not radius, hence my figures approx half yours. My excuse is it is many years since I was Lukes age and did these calcs myself....AND in those days I was using feet not metres, so old figures in head didn't kick in. ( My forte' is excuses)
Cheers,
Dan