Microbiological limits definition

Hi all!
Might someone suggest a formula to calculate the residual limit for microbial contamination in manufacturing equipment for non-steril production?

Up to now, we have applied the following formula:

R.L. (cfu/cm²) = W.S. x B.S. /* S

Where:
W.S. = purified water alert limit (70 cfu/mL) BS = batch size of next product S = contact surface area (cm²)

Do you mind that the formula is correct for the purpose? Might someone have advices to get mes over this item or some useful bibliographic reference?

Many thanks to all
Sheila