What is the minimal volume that has to be collected for particle or germs counting in clean rooms grades C and/or D?

Is there any regulation or recommendation for defining the minimal volume that has to be collected depending on the volume of the clean room area? How optimize the air sampling process by collecting air volumes as small as possible and then by extrapolating to 1m cubic or 1foot cubic for expressing the results according to the local regulations?According to EU-GMP grade A is the only grade to sample at least one cubic meter. For the other graded areas we can use a calculation that is given in annex-B of ISO 14644-1. This basically states that when the sample to be tested would be at its limit we should at least count 20 particles of the largest size concerned for the given class.The calculation reads: Volume equals 20 divided by the number of particles per cubic meter times 1000 (the volume calculated is given in liters). When applying this for a grade C room at rest (2900 particles of 5mu or larger) then this calculation tells us, we have to sample at least approximately 7 liters.