You can calculate the compression ratio using the ideal gas law, CR=((200+14.7)/14.7)^(1/1.4) but valve overlap and timing all influence how much air is in the cylinder at the start of compression at a given engine speed.

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Using that system my car has a compression ratio of 11.22 to 1 I think not.

165psi / 14.7 =11.22

So What Cranking Pressure Can We Expect To See Then?
For an average road engine we don't need to calculate the effect of all the above factors because the expected cranking pressure will be quoted by the manufacturer. Haynes manuals and other tuning guides also quote the figures. If the data is not available you can make a good estimate of the expected cranking pressure from a healthy engine just from the compression ratio. You can expect the cranking pressure in psi for a road engine with a standard cam to be about 17 to 20 times the value of the compression ratio. So our engine with a CR of 10:1 should generate between 170 and 200 psi. The better the condition of the engine the closer the figure will be towards the top end of this range. If the engine has been modified by fitting a different cam then you need to make some allowances. Fast road cams will reduce the expected figure by 10 psi or so. The best thing to do is take a set of compression readings as soon as the engine has been built so that you know what it generates in good condition. For competition engines it's even more important and I recommend to all my customers that they do a compression test after every race to make sure that nothing inside the engine is going wrong. The vast majority of them take no notice of course because it involves a bit of work. Finding a potential problem before it leads to an engine blow up can save a lot of time and money in the long run though.

O.K.
There's 2 ways you can measure compression.
1. Atmospheric pressure is roughly 14.7 PSI. So if you divide your 205 by 14.7, you get a 13.9:1 compression...... As someone else said 'Is this a diesel'? This method is a little bit inaccurate as you loose a slight amount of your air pressure through your piston rings. Also this all changes when you add a Turbo. You have to work of inlet pressure, which of course changes with a Turbo.
2. The right way - Measure the volume of the combustion chamber at bottom dead centre & at top dead centre. Divide the 1st number by the second & you have your ratio. Most guys do this using liquid of some sort, as combustion chambers can be all shapes & sizes.... Difficult to calculate volume.

I've never heard of people calculating CR from cylinder pressures.

It would depend on how much overlap you cam has. Maybe you could approximate the dynamic CR using the cylinder pressure, but probably not the static CR.

10.1 is not the max it is just the best all rounder for a street engine. 10.5 is still ok but you have to be a little more careful. Cold intake temps etc for hot driving days. Also you can retard the timing a little but you will lose the gain of the higher comp. Fuel injected engines can run good compression ratios because the air and fuel is mixed precisley plus alot have anti knock sensers to stop any pinging.

I have around 10.1 and the main difference I noticed was and increase in torque over standard.

It actually depends more on the sqish/quench.
I read a thread and linked it here a ways back that went into the dynamics of combustion.
A bloke reckend that his 440 had higher CR than his mates one, but his didn't ping 'cos his sqish was tighter.
It all gets quite complex.
My G/F's vtec crx has 220 psi, and it runs sweet on 91 octane gas.

No the holden ran fine, no pinging & the motor lasted ages. I'm not sure what factors influence PSI but I don't think it is just the compression ratio. I have seen motors with identical pistons, head etc. run 20PSI different & both run well. Generally though the one with the higher PSI is a rocket (as long as its not a grenade)

At that ratio i could imagin a fair bit of ping-da-ding-ding....

To high for my likeing even in a methanol machine.

My 2 cents anyway

Before you panic, how good is your compression gauge. Some are great but some are woeful. Even though 205 PSI is high, I had a mate with a 202ci Holden red motor running similar figures & it ran fine on super petrol. Is the head alloy on the A15. If so apparently they cope better with higher compression.