My Galaxes have about a 1/4" spacing between backs, but you could even get by with 1/8" if you wanted to.

I agree with John. 

Ken

"The dulcimer sings a sweet song."

Thanks John. What is the spacing on a Galax double back?

Don't overthink it, Richard.  I have not run across any standard spacings myself.  I'd say a space of anywhere from 1/4" to 3/8" would be fine.  You don't need more than that.

Thinking about a possum board, Does anyone know if there are any standards for how much space between the dulcimer and the possum board?  Any science to it?

I have seen what seems to be general recommendations for 3/8:" to 1/2" spacing. 

What are your thoughts?