formula i don't know, but i know a way.
- with the valves closed place an acryclic plaque with a tinny hole to fit a syringe tip into it;
- fill a graduate syringe with a determined amount of engine oil;
- fill up the space below the acrylic with it;
- do the math and subtract the leftover amount in the syringe from the initial one.
The result will be the exactly volume of the chamber.
where i am we use a different metrical system, litres instead of gallons, our syringes are graduated in "ml" (mililitres); ml are equal to cc, simple as that.
hope you were elucidated.