Suggested "solution" for the OP's query.
Metal detector (MD) - Choice of reference test piece/sphere on "risk-basis"
Following is a generic response to the requirement of 4.10.3.4 with respect to “basis of risk” element. (Other specific Regulatory/ “Best Practice” values for given Industries may be equally audit satisfactory if validatable).
The Regulatory answer as to what size of metal contaminant in a food is acceptable is typically "zero".
For a metal detector CCP, attempts could be made to set the critical limit (CL) as the absence of metal in the product. However, absence is simply not practical, as the sensitivity of the equipment will limit the size of the test pieces that are appropriate, ie detectable, by that particular metal detector. (Sensitivity is typically expressed as the minimum sphere diameter that can be detected “100%” of the time).
Nonetheless, while accepting the above limitation, it is evident that operating the MD at its maximum sensitivity for a given machine set-up/food matrix should minimise the risk of any metal passing into the finished product (as far as the MD is concerned).
Accordingly, in order to assure the consumer a maximum safety level (with respect to specific metal contaminants), the critical limit for a given, reference, test piece material will be the smallest size that can be consistently detected/rejected by the MD. A procedure to determine this optimum test piece was previously posted by Marshall/mgourley and IMO is a reasonable response for this requirement (see the 3rd link in Post 11 above).
(the operational portion should probably be expanded somewhat as detailed in,for example, the Tesco MD methodology -
http://www.ifsqn.com...metal-detector/.
(Note1 - there are other factors, eg food matrices, which may cause the CL to vary with different product type).
(Note2 - the above assumes that the MD is well "maintained" and not an antique inasmuch as the result should obviously be "realistic" in the context of, for example, typical customer specifications for retail packs. 20kg sacks may require some further thought
)