A compression tester won't work.

The most accurate way to figure it out is to "pour" the engine. You will need a CC buret for this.

You need to have everything ready for your final assembly. Install the crank, one rod, one piston, with the top compression ring installed. Make sure to put some grease on the ring. Next, install your cylinder head with a head gasket and torque it down. Preferably with a gasket that has been compressed already. Then, tilt the engine over so the spark plug hole is facing up. Fill the cylinder through the spark plug hole with fluid, making sure to get all the air out.

When you get that number, let me know what it is, and I will be able to tell you what your compression ratio will be.