Clean Up rate calculations

i have a confusion regarding recovery rate calculation test defined in ISO 14644-3, where there is a given formula to calculate the rate and its sounds good when i do calculate the rate in air changes per hour, but the confusion is in the statement where it is stated that" cleanliness recovery rate is obtained from the slope value of the line" and in a very next line it is stated that cleanliness recovery rate b/t two measurements is calculated from following formula"
please let me clear what exactly it means that and how could i obtain my rate through slope value.

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