Ok, here's what you need to know to figure out your compression:
Volume of combustion chamber in head.
Measure compressed thickness and diameter of "fire ring" on head gasket.
Volume above the piston to the top of the block when the piston is at TDC. If you have a flat top piston, you can just measure how far down the piston is from the head deck of the block, and then get the bore diameter. You can just use the stock stroke if you don't want to measure it, but measuring it will be fine.
Then I can walk you through the calculation or do it for you.
Also as far as measuring the volume of the cylinder at TDC while the engine is in the block, you can make a cover plate as I described in the link to my thread on how to check compression. Just put the fill hole to where it will be at the top of the block - even if it is not flat. A little "slant" is good also as it makes it easier to tell when the cylinder is full.