I'm glad you got it working, but may I suggest that there's a better way to do it.
First, use the fields in Calibre as they were intended. Store only the title in the title column, and the series info in the series column. Trust me, any time I've tried to work around Calibre's way of doing things, it's always come back to bite me eventually.
Second, use a plugboard to change the metadata on the fly as you send it to your device. Set up your plugboard template once and never worry about it again, instead of messing with mobi-to-mobi conversions on books you add in the future.
Admittedly, I can't help you with setting up a plugboard template because I don't use plugboards myself, but there are plenty of capable members on this forum that do. I just know that this feature is designed specifically to do exactly what you're asking.
By the way, to get started with plugboards, see
here.