How to calculate compression
Compression from UT is interesting to estimate knock limit
Compression from upper edge exhaust port is interesting for efficiency
The compression ratio from upper edge exhaust does not lead to the distance to the knocking limit. The knocking limit is determined by the pressure at the end of the compression phase and
this is determined by temperature, filling and compaction.
So if you increase the upper edge of the exhaust port with the aim to improve the filling at high speed, nominally the compression
ratio falls but the pressure in UDC increases and the engine comes closer to the knocking limit. In the case of compression from UT you have at least a rough guide to compare engines of
different tuning stages.
Very probably the compression ratio calculated from upper edge exhaust port makes sense when you are dealing with the thermodynamic efficiency. The deeper the edge is, the longer pessure pushes the piston down. Greater efficiency means at first less consumption and second higher performance.
With four-strokes engines the compression ratio is calculated from BDC, even though racing engines also open outlet 80 degrees before BDC and close inlet 70-80 degrees after BDC.
The differences to the two-stroker are thus not so large.