I’ve made several of these over the years. I’ll second Mr. Arnold’s
number of 1.5 sizes (US). This is for a standard commercial wedding
band 1.5 mm thick. A thicker band, i. e., used for setting diamonds,
will require a larger size shift. I can’t figure out where the “5
sizes” increase comes from, unless it’s using a different sizing
system not used here in the US. That means a finished ladies size
six ring would have to have size eleven bands. This sounds very
unrealistic to me.