A US quart is 32 oz.  So the math goes like this, 32 oz (w/ 8 oz nitro) + 10 oz (w/ 1.5 oz nitro) = 42 oz (w/ 9.5 oz nitro), or 22.6% nitro.  (That's 9.5 divided by 42 times 100).

The castor/synthetic numbers are irrelivant to the discussion of nitro content.

Phil