Thanks for all the info.
Now I remember where I read about priming with grape juice:
Steve Pauwels Wheat Wine Recipe p.144 - "Brewing with Wheat" by Stan Hieronymus.
"Bottle with grape juice to increase complexity...consentrated grape juice has 68 degrees Brix or 68 grams sugar per 100 grams solution."
The degees Brix lines up with Williams Brewing Muscat Concentrate ".. with minimum heat under vacuum to 68 Brix."
So I will start here with a Brix conversion to figure out sugars to volume but will need more coffee to figure out "68 grams sugar per 100 grams solution".
If I do this I will split my 5gal batch - half sugar, half grape consentrate