For a fire damper 600 x 600 mm with L = 500 mm.

Ps = 1000 Pa f = 0.003
øL/app = 0.003 x 1000 0.65 x (2x 0.60 + 2x 0.60) x 1 = 0.641 l/s

This fire damper may therefore leak a maximum of 0.641 l/s into the surrounding area at 1000 Pa in order to comply with airtightness class C. Based on the formula above, the maximum permissible leakage for a fitting (with a length of less than 1 m) in airtightness class C at the following pressures can be calculated as follows:
500 Pa øL/app = 0.17 x (2H + 2B) or 0.17 x (π x D) 1000 Pa øL/app = 0.267 x (2H + 2B) or 0.267 x (π x D)

Download the EN1751 Test Report Casing Air Leakage Rate here.