You said one of the wall shiething layers is "soundboard".
In that case, the mass of soundboard is almost nil. In fact, since its an absorption product, your wall is probably the equivelent of a little more than one layer of drywall. So, I would guess that the 1/4" second pane would be about right. To be on the safe side though, I would submit that 3/8" would be better.
Not to rain on your parade, but I wished you would have asked here prior to putting that "soundboard" up. When it comes to Transmission Loss, Soundboard is NOT Mass. They refer it as Soundboard when it comes to ABSORPTION, used on the interior of a space. Oh well, done deal. Anyway looks great and I hope you get the iso you needed. Keep posting pics. Always like to see people finish their projects.
fitZ