Hi Tech and Denny,
I didn't measure the deck height extra. I did the measuring like this:
First I smeared about 3cc of grease round the outer edge of the piston, so as to get a water tight seal (the first measurement went wrong here because water escaped past the rings).
Then I screwed a lexan plate with a vent hole onto the block, sealed with grease.
Then I filled the cylinder with water from a 100cc burrette til I was at the vent hole, which was at the highest point.
Surely that would already include the deck height then?
So it's 761cc including deck. The gasket I calculated like this: it has 1mm, so 0.1 cm of height (that is about .04"). The radius is 5.2 cm. And radius squared times Pi times height is the volume. so I get 0.1*5.2*5.2*3.14=8,5. OK, Sorry about that (Woops!) my mistake, didn't think I'd make a mistake that stupid, after all, I'm studying math
That would drop me to 8.4:1 CR ???????
Can only be a measuring error in the 761...
I'll get back to you as soon as I find out my mistkae, but it's getting a bit too trial-and-error like for me here

PS: Of course i can alwas go wrong here: I filled the 100cc burrette 8 times in the process. And if I can only fill it to let's say 2 or 3 cc exact (or the burrette's measurings include the snout) I'll be doing the same mistake EIGHT times. If that's 2 cc's every time I'll be looking at a error of 16cc :-(