340 compression test

Picked up a 340 with X heads, completely unknown build.
Did a compression test on my 340 and got what I think are very high results.
The engine was on the floor of the shed with a 904 and starter bolted up so I could crank it over.
Has a tunnel ram with no carb on top.
I can see domed pistons through the spark plug holes so was thinking that this may be around 10.5:1 compression but just don't really know.
All figures from the test were obviously done cold and with no oil squirted in the spark plug holes.
220psi to 240psi on all cylinders.

This seems a bit high to me.
Any ideas what a 340 with 10.5:1 compression in good condition reads on a compression test?
Is there any way to calculate the compression ratio from the psi readings I made knowing that this is a 340?

Comments welcome!