What product do you produce? I always find it hard to validate magnet depending on the criteria you used.
But anyway, here are my thoughts on it and the process I will recommend to use.
In my experience we always use limit for foreign material to be as low as technically possible but it should not be greater than 2mm (for the size), for other limits, we use for sieve that it should be intact and correct fitting and all product must flow through the sieve. For magnet, same, correct fitting and the magnet strength should be equally distributed between housing and magnet bars. Usually, it is recommended to have 2 layers of magnet to consider equal distribution and maximum retention given the flow of product.
Before implementation, we validate the plan, for example if we say that this should be inspected every 8 hours, then our production run should have window every 8 hours allotted for inspection which means the set-up must be capable to be easily dismantled for inspection and cleaning. That there should be sufficient lighting and space for inspection and dismantling. For magnet, it should also be aligned with your operating condition (e.g. temperature). For sieve, if you use it in combination with magnet, it is preferred to be the magnetizable type. As if not, what benefit is the installation of the magnet add?
AS for the sizes and magnet strength, we request LOG from supplier. For the sieves, we measure opening randomly for 30 openings using caliper using certified test balls or rod. For magnet we use gauss meter individually and between magnet. For the gauss meter, we usually oustource that one.
For magnet we then allow to pass product containing the identified contaminant and pass through it for 30 times
AS for verification of the equipment, we repeat the measurement every year. AS for verification of the system, we check inspection record if it is being done as per plan.