It is essential to know the compression ratio of your engine. On a naturally aspirated engine it is essential to have the correct compression ratio if best performance is to be achieved, on a Turbocharged engine it is essential if piston damage is to be avoided.
To high a compression ratio & the engine will quickly run into detonation. It is essential to avoid this as it can take literally seconds to completely destroy a piston.
This point cannot be stressed enough !
Fuelling, ignition, fuel octane & intake temps all play a part in how much boost can be run on a given compression ratio but the figures below will give a basic guide.

If the engine is set up right on the limit it will only take a hot day, or a bad tank of fuel to run the engine into detonation.
For a safety margin it is better to run a lower compression ratio than is required for the boost run.
The slight loss in bottom end torque & fuel economy is a fair trade off for an engine which will stay free of detonation.

9.4-1 Standard Metro Turbo compression ratio. Suitable for standard 7psi of boost, a little more if an efficient intercooler is fitted.

9.0-1 Suitable for about 10psi or so. 

8.5-1 Suitable for about 15psi or so, efficient intercooler essential.

8.0-1 Suitable for 20+ psi Efficient intercooler essential. Undercrown jets advantageous.

Don't take these figures as gospel, as listed above other factors play a part, but as a basic guide they should be sound.

To do the calcs it's V+C divided by C

V is the swept volume of one cylinder, so in the case of a 1275cc engine you divide by 4, this gives 318.75cc This is the volume of one cylinder.

C is the volume above the piston at TDC. So this includes the head, head gasket, piston dish, piston ring land and any space btween the top of the piston & the top of the block if the piston doesn't come flush/level. You need to add all these volumes together to give C 
The standard head is 21.4cc 
Turbo head Gasket 3.8cc
Turbo Piston dish 12cc
Ring land 0.8cc

You then need to measure the distance from the top of the piston to top of the block when the piston is at TDC using a DTI.
Unless the piston comes completely flush there will be quite a bit of volume here.  
The following chart will give you the figures for a standard 1275 bore, +0.020" & +0.040" overbores.

If the piston sits further down the bore than the table shows, the figures can be added together to give the correct distance.

Once you have the figures you can do the calculation.
V+C divided by C=