You'll calculate the minimum ws dimension required based on tension/compression on the diaphragm plate, at the face of the HSS column, where the cut-out for the column begins. Then, it appears that for detailing convention, this dimension should also be projected the other direction, parallel to the beam centerline, to determine when you can start trimming the diaphragm plate.
I don't see this connection discussed in the new AISC Design Guide 24, which deals with HSS connections. However, it is discussed in the old black HSS Connections Manual.