How to calculate staircase pressurization ? Is it like substracting stair occupied volume from total staircase volume and multiplying by ACH ?

As per British standard BS5588 , There are 2 ways of calculating this procedure 1 is pressure differential between each room and must have pressure differential sensor or switch (+50 Pa.) and other method is calculating 2 doors open with velocity of smoke 2 m/sec.

As per Singapore std. staircase  calcs are using 6ACH in normal mode and the other is using 12 ACH during Emergency mode. This is include smoke stop lobby (SSL).

Thanks