2 strokes, are by design, a variable compression engine. It's largely the action of the exhaust /expansion chamber that affects this.
Static compression is calculated as SV+CV/CV. Where SV = Cylinder Swept Volume and CV = Head Compression Volume.
For arguments sake:- Assume SV = 240cc and CV = 25cc.
240+25/25 =10.6:1
10.6 x 14.5038PSI (1 atmosphere of pressure) = 153.74 PSI.
153 PSI is a figure you would commonly measure in a 4 stroke engine. NOT a 2 stroke. Why?
The 2 stroke only starts to compress its contents once the exhaust port is closed when carrying out pressure tests.
So, again for arguments sake:- Assume the exhaust port duration = 180 degrees, therefore there is now only the other 180 degrees left available of compressible volume.
So, this effectively means the engines usable compressible volume is only half the Swept Volume = 120cc.
120+25/25 = 5.8:1
5.8 x 14.5038 = 84.22 PSI.
Using most 'off the shelf' compression testers will introduce an error here because you have to consider the fact that the plug hole adaptor, the connecting hose and the Bourdon tube within the gauge itself has volume. This volume becomes part of the Compression volume of the head. So, your 25cc head is now maybe 26cc? The reading you get will be lower than it really is..........
For this reason I use an electronic 20 bar Peizo sensor in the plug hole and measure the voltage output with a bit of software on a laptop (Anal.... but damm accurate)
Squish isn't really associated with stroke. It is however associated with conrod length. Assuming the use of a good quality conrod, then 1% of the conrod length is a good guide to minimum squish measurement which would be 1.07mm for a standard length rod.