It will be accurate to maybe plus or minus a cc or so. I wouldn't use it to equalize chamber volumes- you need better accuracy for that. But for getting an estimate for figuring compression ratios, it's fine.
I don't know if you were going to smash the clay down into the beaker to measure the volume, but the easier way is to add a known volume of water to the beaker- enough to cover the clay- and then ease the clay down into the beaker and see the lever the water rises to. That level, minus the starting amount of water, will be the chamber volume in cc.