Don't all 60 series 'Cruisers need the 3.5" backset? (depth from inside rim to mounting surface)
If I read this correctly the +15mm offset would be mid-line (half the width) plus 15mm (.60").
Wouldn't that make it something like 4.10" backset? You might be able to do it w/ a 1/2" or 15mm spacer.