The best rationale is to take the data you have from the certification and show this on a floor plan; this should prove that, based upon a risk free study of the cleanroom, you have a fairly uniform distribution of particles throughout your room. This means that it would pass the ISO14644 requirements for an ISO 5/7 room.
Now, look at the workflow patterns in your cleanroom; from this you will be able to see where the personnel traffic and material flow occurs in your room. This is the risk information you need. If you overlap the certification data with the flow patterns you will see where the risk interacts with the particle distributions and can select suitable particle counting points based upon Risk and utilizing the cleanroom data to prove justification of location.
Ultimately, a second study of this are will be required as the fixed probe can not be installed in a floating location where the portable data was gathered so a suitable location that allows for the work to take place and quantifies those particles can be selected.
Read Choosing the Most Suitable Non-viable Sample Point Locations for more information.